Shrines are some of the most useful items in the game. You could actually save thousands (even hundreds of thousands of gold) if you use them correctly. They're some of your most powerful support stuff. Of course, there are the "bad" (they're not exactly bad, as will be discussed below) shrines which you don't want to touch. So don't touch every shrine you encounter either.

Touching/using the correct shrine at the right time could save you a lot of time, gold and life. Shrines can be classified into:

 a) affects player attributes (STR/DEX/MAG/VIT)
 b) affects player inventory items (gold, item durability, staff charges, potions, AC/damage[single-player only])
 c) affects player spell ability (spell level)
 d) affects other players

Shrines are only found in church and catacombs. All shrines are one-use only. Its power disappears after it's touched once. All shrines will display a corresponding message when touched.

The name of the shrine, its effects and message displayed are listed below. I've included my own usage notes too.

Affects player attributes:

CREEPY +2 STR "Strength is bolstered by heavenly faith"
ABANDONED +2 DEX "The hands of men may be guided by fate"
EERIE +2 MAG "Knowledge and wisdom at the cost of self"
QUIET +2 VIT "The essence of life flows from within"

Tip: use the above shrines freely. They won't increase your attributes higher than the maximum allowable for your character though. In fact, you want to hunt for these shrines at higher levels (saves you from buying 2 elixirs worth 5000 gold each from Adria)
(refer to Jarulf's Guide for maximum stats allowable for each character class)

MYSTERIOUS
One attribute raises by 5 pts, the other 3 attributes drop 1 pt.
(overall, you still gain +2 attribute pts net)
[won't raise higher than character limit: will subtract 1 pt. from the 3 attributes, even if can't +2 pts to the other (maxed-out) attribute]
"Some are weakened as one grows strong"

Tip: Use this if your attributes haven't been maxed out yet.
(I almost always use it when my attributes haven't been maxed out yet).
Don't use this if your attributes have already been maxed out.

You might not want to use it depending on the circumstance:
e.g. your STR is just enough to carry this cool king's bastard sword, and if your STR were lowered, you'll not be able to use it anymore. (although when I'm still playing at lower levels, I usually keep several attribute-modifying rings so I could juggle them around. If you've got a few spare level-up points, that allows you to re-balance your distribution of attribute points too.)

Note: the attribute that increases by 5 pts. is random so you won't know which it will increase. However, it's predetermined when the game is generated (ie. in single-player games, if you approach that shrine, it always have the same effect when you load/save the game)

Affects inventory/items:

ELDRITCH
All potions (healing/mana) in inventory (including belt items) become rejuvenation potions of the same type (partial/full).
"Crimson and azure become as the sun"

Tip: Use this freely. Better yet, use this when you've just collected a lot of red/blue potions, esp. the big ones which transform into those expensive full rejuv potions. (you could actually town portal first to Pepin/Adria to buy full healing/mana potions and then use the Eldritch shrine; you could always sell the transformed gold potion back to Adria for the same amount of gold you bought it when it was a full healing/mana potion [but why would you do that?]).

SPIRITUAL
Each empty inventory box is filled with a certain amount of gold. (up to the forty boxes -- at dungeon level 3 or 4 [I emptied all slots], I got about 400 gold total; in dungeon level 13, I got around 1500 gold)
"Riches abound when least expected"

Tip: Use this freely. OTOH, you might not need to use this since the amount of gold given to you is not much (esp. when you're in nightmare or hell modes where gold is abundant). Plus since the gold is scattered all over your inventory slots when you use this, you'll have to re-arrange those small amounts of scattered gold in your inventory slots again to make room.

GLIMMERING
Identifies all 'not identified' items in inventory.
"Mysteries are revealed in the light of wisdom"

Tip: Use this freely, especially for lower level players where 100 gold means a lot. Better yet, collect all your unidentified items together when you visit this shrine to save hundreds of gold.
(note that all items, whether equipped or unequipped, are identified so you might want to make full use of this fact)

Note: You might want to have those unidentified items repaired first before having them identified, because it's almost always cheaper to repair a non-magical item (repair cost depends on 3 things: a) the cost of the item, b) the durability of the item, and c) whether the item is magical or not. Almost the only time where an ID'd magical item is cheaper to repair is if it's an "of structure" item which has high durability (assuming the "of structure" item doesn't also have a prefix.) "of structure" items are usually easily noted because their durability is odd-numbered and unusually high for that class of item. See discussion of repair costs in the appendix below.

For high-end magical items (e.g. you found that item in hell), you almost always want to have that item repaired first before getting it ID'd, if you have use for that kind of item. Note that Unique items are always at full durability when found (it may not be fully recharged with spells though if it's a staff -- unlike normal/magical staffs, which are always fully charged when found. The spell recharge cost for Unique items doesn't seem to change whether it's been ID'd or not yet ID'd though.)

RELIGIOUS
All items in inventory (equipped & unequipped) are fully repaired.
"Time has no effect on the strength of steel"

Tip: Use freely. It's better to pool together all your expensive equipment first before you use this to save possibly thousands of gold. You might want to use this just before quitting playing the game (if you recall where this shrine can be found) so all your items are fully repaired at once at no cost. Of course, you don't want to wait until the red "needs repairs badly" icon appears in the corner before using this shrine.

HIDDEN
One item in inventory loses 10 DURability, other items increase by 10 DUR. (Only items equipped/worn on the body were affected; the item that lost 10 DUR is random. If the item's current DUR is 10 or less, it's DUR becomes 1; if the max DUR is 10 or less, them item's max DUR becomes 1)
"New strength is forged through destruction"

Tip: One of the most useful shrines in the game. Must use this correctly though to gain the most benefit. DURability affects repair cost. An item with higher DUR will have a lower repair cost (esp. with those high-end items which easily cost thousands of gold to repair several DUR points). {it seems the new lowered/increased repair cost is not immediately reflected when you have that item repaired by Griswold -- you might have to drop the item in town or dungeon and re-equip it (or restart new game) before this takes effect -- please verify. The important thing is that this takes effect sooner or later though.)

Since the item that decreases DUR is random (but pre- determined by the game; in fact there is a predetermined order of which items' DUR get decreased -- refer to Van_Damned's experiment discussed some time ago in the DSF), you want to ensure that you have four items equipped. that way, you gain a net 2 items increase DUR each time you touch a "HS" (hidden shrine):

(Four slots: helm, armor, 1-handed weapon, shield: 3 increase = +30 DUR, 1 decrease = -10 DUR ==> +20 DUR net)

Do not equip indestructible ("of the ages") items since they're not affected by the shrine. And the shrine will surely decrease one of the other items. A 2-handed weapon (eg. great axe, bows) is harder to increase DUR since:

(Four slots: helm, armor, 2-handed weapon: 2 increase = +20 DUR, 1 decreases = -10 DUR ==> +10 DUR net, but 2-handed weapon has 2 chances of decreasing so it's a bit of a gamble to have it increased as well as decreased.)

(side comment: in my encounters, it seems slightly more often shields are the items that decrease 10 DUR, which is why I often buy a non-magical buckler or shield and equip it whenever I touch a HS. This is not conclusive though. Van_Damned says helms are slightly more often decreased for him though. Maybe all four slots are equally likely, noting that 2-H weapons are 2 times more likely if these were the case.)

There are entire threads regarding the use of Hidden Shrines. There was an interesting experiment done by Van_Damned before, which is informative. And a few others have tried increasing an item's durability to exactly 255, which I think made the item 'indestructible'; otherwise, the item's durability "rolls over" to 1 to 10 DUR again.

STONE
All (spell) staffs in inventory are recharged.
"The power of mana refocused renews"

Tip: Use this freely. Better yet, collect all your staffs that need recharging so all of them get recharged at once. You might want to note the location of the stone shrine, use up all your staff's charges and then return to this shrine for full recharging.

DIVINE
Gives 2 full rejuvenation potions, or, 1 full healing and 1 full mana potion; plus fills up both healing and mana balls.
"Drink and be refreshed"

Tip: Use this freely. Of course, you want to use this when your life and/or mana is below full, to make better use of this shrine. I think it's random as to whether 1 blue and 1 red potions appear or 2 gold potions. I have this feeling though that at lower dungeon levels, the blue/red appears more often (please verify).

(** note: the 3 shrines below appear only in single-player mode, according to Jarulf's Guide. Also, the effects of gloomy [verify] and weird shrines only remain in the current game: once you start a new game, the +armor or +damage effect reverts to its normal status.

GLOOMY
Adds +2 AC to all armors (helms, shields, etc); reduces damage (-1) to all weapons' max. damage.
"Those who defend seldom attack"

Tip: Use freely. I don't recall how this shrine actually works -- whether only the equipped items are affected or all items are affected. (possibly only the equipped items are affected -- please verify). In any case, you don't want your weapons' max. damage to go down, so you drop all your weapons first when you're going to touch this shrine.)

WEIRD
Maximum damage of all weapons increase (+1).
"The sword of justice is swift and sharp"

Tip: Use freely. This shrine works on equipped and non-equipped weapons. Place all good weapons in the inventory slots before using the shrine for most effect.

THAUMATURGIC
All chests in current dungeon level regenerate with new items. (opened chests are closed again)
"What once was opened is now closed"

Tip: This must be a good shrine too. I did not encounter this shrine yet when I played single-player though. Obviously, you want to open every chest in the current dungeon level first before you touch this shrine, so you get the most number of items.

Affects player spell ability:

ENCHANTED
Minus a level for 1 spell in spellbook, gain a level for all the rest of the spells in spellbook (if spell was level 1, it drops to spell level 0 and becomes unusable)
"Magic is not always what it seems to be"

Tip: This shrine is extremely useful -- it can effectively save you hundreds thousands of gold in one touch (how? let's see: a book of blood star costs 27500 gold, teleport costs 20000 gold, golem costs 18000, etc...) It's best if you've got more than a handful of spells in your spellbook, since 1 spell always gets decreased by a level and the rest increases by 1 level. Once again, the spell that gets decreased is random (but predetermined).

Its importance becomes even more apparent when your warrior or rogue can't read any spell book anymore because they need higher than 200 MAGIC to read. In fact, enchanted shrines are the ONLY mechanism that allow your warrior/rogue to increase their spell levels to level 15 [there's no other way, magic enhancements wouldn't be enough -- see appendix below].

Almost all the higher-end spells need 255 MAGIC to advance to the next higher level, which your warrior/rogue needs badly. (Not so much a problem for high level sorcerors with high MAGIC, though the savings in cost/time still matters).

As long as not all your spells are at level 15 yet, you can hope to benefit from this shrine. Don't use this shrine if all your spells are already level 15 (or only 1 or 2 more spells are not at level 15), since it will decrease one spell by 1 level. (e.g. my lvl 38 sorcerer has maxed out all spells to lvl 15, so he never touches enchanted shrines anymore.)

(Additional note by Van_Damned: if the spell that decreased was level 15, it decreases 2 spell levels and becomes level 13.)

The best case would be if the only spell not yet reaching level 15 (maximum spell level) is your firebolt spell, since you could always equip enough rings/armor to make him up to the required 177 MAGIC to read the firebolt spell level 15 (firebolt has the lowest MAGIC requirement). The only other spells that don't need 255 MAGIC to gain level 15 are Heal and Heal Other which need 192 MAGIC to gain level 15 ability.

Note that not all spells need level 15 for maximum ability. You want to gain either minimum mana cost and/or maximum damage for your spells. Note that rogues reach minimum mana cost much faster/earlier than warriors and sorcerers since their spells' mana cost is 75% of normal (but never goes lower than the minimum mana cost for that spell). (I first noticed this when my rogue learned a level 1 golem spell and she needed only 75 mana to cast, whereas my warrior and sorcerer had needed 100 mana to cast a level 1 golem spell.)

FASCINATING
Firebolt Spell gains 2 levels (or added), but maximum mana goes down by 1/10. (amount of mana subtracted seems to be 1/10 of total base mana [when not wearing any magic enhancements], rounded up)
"Intensity comes at the cost of wisdom"

ORNATE
Holy Bolt Spell gains 2 levels (or added), but maximum mana goes down by 1/10.
"Salvation comes at the cost of wisdom"

SACRED
Charged Bolt Spell gains 2 levels (or added), but maximum mana goes down by 1/10.
"Energy comes at the cost of wisdom"

Tip: Do NOT use any of these 3 shrines. They permanently decrease your maximum mana. These are especially detrimental to sorcerers whose lifeblood is their mana. The higher mana you have, the bigger you lose.

The only possible time to use this shrine that I could think of is when playing single-player mode and your character is still at very low-level with little base mana (eg. lvl 1 warrior). You gain the above spells quickly (and get to cast them at minimum mana cost quickly) which can offset the slight (but permanent) mana lost in gaining the spell to a higher level. (I still probably won't use them even when playing single-player mode.)

MAGICAL
Mana shield spell is cast on you for free.
"While the spirit is vigilant, the body thrives"

Tip: Use this depending on the circumstances. Obviously, since a warrior has little mana, the warrior might not want to have a mana shield on, preferring to use his mana to cast heal (more efficient use). Sorcerers (esp. lower-level ones) would probably like to have a mana shield over their head. Higher level sorcerers probably already have a mana shield activated when they encounter this shrine though. For rogues -- it depends on whether they're in "warrior" or "sorcerer" mode.

CRYPTIC
Nova spell erupts from the player character when the shrine is touched (no damage to player, but damages monsters) and fills up player's mana ball.
"Arcane power brings destruction"

Tip: Use freely. You want to use this when your mana ball is not full. I'm not sure if other players are hurt by the nova spell (please verify). At lower levels, if you enter this room and it's full of monsters and you're able to reach the shrine, you could actually kill all the monsters in that room with the nova spell and refill your mana ball at the same time. (You could also use Telekinesis to "touch" this shrine and cast a free "Nova" spell emanating from your character.)

HOLY
Teleport you a short distance (like phasing spell).
"Wherever you go, there you are"

Tip: I think this is one of the useless shrines. What it does is cast a phasing spell on your character, which typically just moves you outside the room. Phasing location is random, so you might want to use it to move across a room on the other side of the wall instead of having to walk a great distance around -- you're lucky if it puts you where you want to go.

[Ok, the 3 shrines above didn't affect player spell ability, but spells were involved. If not, consider them misclassified].

SECLUDED
Automap is completed for current map level.
"The way is made clear when viewed from above"

Tip: Most of the time, I don't want to use this shrine yet. Because if you want to be sure you completely clear a level, you want the unexplored areas to not yet appear on the overhead map. Once the automap is completed, you could forget which part you've explored and which part still has monsters (where to retreat, for instance, unless you have a photographic memory).

On the other hand, if you're not planning of clearing the entire level or you're just hunting for other shrines or the location of the stairs, this is a good shrine to use first. Other shrines are usually located in corner rooms or at the side (in the church) and shrines are always inside enclosed areas in the catacombs. So if you're hunting for enchanted or hidden shrines, you know at once where to look from the completed overhead map and just teleport your way there.

Affects other players:

SPOOKY
Other players are fully healed.
(user) "Where avarice fails, patience gains reward."
(others) "Blessed by a benevolent companion."

Tip: Looks like a good shrine to use, esp. when your companions are heavily injured. When you're the only player, there's no effect in using this shrine.

TAINTED
User gets +1 attribute, other players get -1 attribute.
"Those who are last may yet be first"

Tip: (Sorry, I've not played multiplayer much with other players, so I haven't experienced what this shrine really does yet. The listed effect is from Jarulf's Guide.) By the listed effect, it appears the user is benefited while other players are not. When you're the only player, there appears to be no effect to the user though.

Miscellaneous:

Goat Shrines can be found as high as lvl 4 (only when that lvl has goat demons/flesh clan) up to lvl 8. (some say they can also exist in the caves, but I've never seen one there). They're typically guarded/surrounded by goat demons (and it seems more often than not, these are the archer-type variety of goat demons).

Cauldrons can only be found scattered in hell, lvls 13-15 (lvl 16 doesn't ever have anything there -- no cauldrons, no chests, no items/potions lying on the floor, etc.)

GOAT SHRINE/CAULDRON
Random effect similar to the other shrines. The message and effect go together though (effect as the above shrines).

Tip: Goat shrines and cauldron's effects are random, but predetermined (it doesn't change its effect). Since it's random, I never touch them in multiplayer for fear that it might be one of the "bad" shrines. (I don't want to take any chances of that happening.)

In single-player mode, it's safe to do so: just save the game first, then touch the shrine -- if you don't like the shrine's effects, just re-load the game and don't touch that shrine/cauldron.

Pools and Fountains can be found in the church, catacombs or hell (lvls 1-8, 13-15). (please correct me if you've seen them in the caves). Some are one-time drink but others allow you to keep coming back for more.

BLOOD FOUNTAIN
Restores 1 point of life each time you click on it.

PURIFYING SPRING
Restores 1 point of mana each time you click on it.

Tip: These multiple-use fountains are good for any character. Just keep clicking on them to recharge either your life or your mana.

Purifying springs are even better: because you could then cast Heal to refill your life too, then drink again. Or you could cast mana shield, then refill your mana. Or cast golem, guardian, etc. (which require lots of mana).

Unfortunately, one click = one point. so it may take a very long time to refill the 800 mana of your lvl 38 sorcerer; assuming that your mouse button doesn't break from your furious clicking; and you've got untiring fingers. Killing a monster, taking its gold to town via town portal, buying a potion to refill your life/mana might actually be less time-consuming if you've got lots of life/mana].

Now I won't suggest you to keep drinking while monsters are attacking you unless you have high AC/resistance. Kill the monsters first and drink later. (should save you mouse clicks)

MURKY POOL
Casts one free Infravision Spell.

Tip: Use freely. To make full use, use it when you're ready to explore unexplored terrain. Infravision allows you to see monsters behind walls, etc.; but only for a limited amount of time. (I think the time becomes longer as character's level goes up -- please verify).

FOUNTAIN OF TEARS
Raises one attribute by one and lowers another by 1.

Tip: No need to use this. Net effect is +0 (unlike mysterious shrine where you gain +2 attributes net). You might attempt to use this if you badly need to add one point to, say, your STR to be able to wield that great axe and don't mind losing, say, one MAG point. But then, you're trying your luck here: the shrine's effect is random and it might not do what you want.

I almost never use this shrine. Try it if you must. Don't use this shrine when all your attributes are already maxed out.

{Addl note by Van_Damned: you can also use telekinesis to "touch" shrines, eg. in case you don't want to disturb the monsters inside the enclosed room.)

Appendices:

A) Magic-increasing items: (useful to have when reading spell books)

 Dreamflange          +30 magic
 amulet of wizardry   +30 magic
 ring of wizardry     +30 magic
 ring of wizardry     +30 magic
 helm of sorcery      +20 magic
 armor of sorcery     +20 magic
 shield of brilliance +15 magic
                    ============
                     +175 magic 
 

* for a maxed out rogue with 70 magic, wearing all these items gives her up to 245 magic (Jarulf's guide used to list maximum magic of rogue as 235 magic, maybe Dreamflange was not considered as weapons have maximum of +20 magic) -- still not enough to read spell books requiring 255 magic to reach maximum spell levels.

B) Computation of repair cost:

From Jarulf's guide, the repair cost of an item is:
 0.15 * item_price * (1 - current_durability / maximum_durability)

The above formula works for all magical/unique items.

For non-magical items (or un-ID'd magical/unique items), the multiplier is 0.50, which gives: 0.50 * item_price * (1 - current_durability / maximum_durability) Thus, non-magical items are more expensive to repair given the same price, but magical items almost always have more expensive prices, so they cost more to repair. The exception are items "of structure" since while "of structure" items cost 2x the price (+300) of the non-magical type, they have much higher durability, which lowers the repair cost per durability point, plus the fact that the multiplier is 0.15 vs 0.50. Other possible exceptions for the ID'd magical item to have a cheaper repair cost than it's non-ID'd counterpart would be "of craftsmanship" and "of many" items. These types of magical items can usually be spotted (prior to being ID'd) because they have much higher durability points (usually odd-numbered too).

C) Summary

With 25 different shrines (22 if we consider only those that appear in multiplayer), how do you remember which shrine to use? Well, the first shrines I remember are the 3 shrines that I should NOT touch, which are: FASCINATING, SACRED, ORNATE. Remember only these 3. These 3 shrine effects are irreversible. (even the other attribute-reducing shrines have reversible effects -- eg. just go buy an elixir to replenish that lost attribute point.) But you couldn't increase lost mana any way (except cheating programs, which we legit players frown upon).

Next is to remember the good attribute-enhancing shrines: you want to remember which shrine increases which attribute -- esp. if you play with other players so the mage in your party would focus on the magic-increasing shrine (EERIE), warrior on CREEPY shrines, etc. (unless your warrior badly needs, say, 2 more magic points to read this level 3 chain lightning spellbook...). Four shrine names to remember here (see list above) plus the Mysterious shrine.

Next shrines to be remembered would probably be Enchanted and Hidden shrines. These shrines are usually used when your characters are at higher levels and you've found some good high-end stuff. After all, not much use for the enchanted shrine if you just got 1 spell in your spellbook -- only that one spell goes up in level [please verify]. Also, while it's nice, to have increased your magical fine rags to 26 DUR, I'd rather get normal leather armor instead.

If you don't want to "hurt" your other co-players, don't touch the Tainted shrine.

Don't touch Secluded shrines if you want to know which part of the map has been explored and which part still has monsters.

As for the remaining shrines, they're either good or don't do any harm. You might want to remember specific shrines so you know what each does; but no harm if you touch them -- eg. religious, stone, spiritual, divine, etc.

And for goat shrines and cauldrons, don't touch them either: you don't want to risk having maximum mana decreased (even assuming that goat shrines/cauldrons reflect all the possible shrine effects: 3 out of 22 possible shrine effects are "bad" -- that's only 12% chance of touching the "bad" shrines, assuming all shrine effects are equally likely.)

Credits:

Thanks to Jarulf for his excellent Jarulf's Guide; also to Van_Damned for the experiments with hidden shrines and Sourceror for his additional comments regarding HS.

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