Drinking from fountains usually cause more bad things, then good.
You can find rings and amulets.
Sometimes creatures can come out, being friendly.
You can release a spirit, that grants you a wish.
You may gain stat boosts.
You can gain certain magical effects.
Creatures can come out, and be hostile. A lot of them.
Brown Flat Slimes (Usually I die from these... -_-)
Snakes (SNAKES ON A FOUNTAIN.)
Dark Frogs (Varying in size.)
This accounts for the friendly, too.
The Spirit could be angry for being summoned.
The water can be contaminated by vomit, dead creatures, etc, causing stat drops, and even poisoning/disease.
You could get randomly teleported, which gets you soaked in water.
Few things to add here:
1. The magical effects you can gain? If you're stupidly lucky, they can be permanent (this water tasted very odd). This can either be *great* (invisibility, polycontrol, possibly even haste), nice (can't smell anymore), or horrid (slow, teleportitis, polymorphitis). The thing IMO that makes it worth the risk is that the good magic effects seem to outweigh the bad.
2. Diseases include: Lycanthropy (every so often you'll turn into a werewolf with generally higher stats but no equipment and only biting as a weapon), tapeworms (after a certain number of turns or so you'll start taking internal damage which can only be cured AFAIK by vomiting until the parasite comes out), and leprosy (every once in a while a limb will fall off; note that if this happens and you go to the healer for help that the removed limb in question will still be diseased, such that if you reattach it you'll be a leper again). There may be others, but I'm fairly sure those are it (although teleportitis and polymorphitis might qualify under certain conditions
3. Missed an enemy; green conical slimes. IMO these are *the* worst unfriendlies you can get, although the spirit who wants to kill you likely runs a close second judging from my recent genie experiences. The mommo slimes don't seem *quite* as bad if you have decently covering equips, but in any other case their sticky acid will prove just as deadly in the end as the green slimes.
4. Best way IMO to cut the risks down to an acceptable level? Have a helper around; if you are good enough buds with say, Loricatus, he'll send one of his kickass angels down for a time, and they'll be sure to beat the crap out of anything you can't handle. The friendly creatures you can get by drinking from the fountain are also good candidates for fountain protection. If you can defend against the enemy summons, the only other nasty things are the stat drains, diseases, and the not-so-nice magic effects, *most* of which can be fixed/mitigated by other means.
*admits to being a fan of fountains even when he has no hope of killing the summons*