ToME WikiRavenRed: Just a quick idea on what the effects of low sanity might be like if we moved on from the admittedly annoying, but hardly crippling "You stab the Robocop in the throad" type messages.
These effects might occur with a frequency of ((41-WIS)/2:1 ) in (Current Sanity/Max Sanity) * 100) if your sanity is below 100%. This makes them annoying, but not absolutely inevitable.
Random use of items - any attempt to "quaff", "read", "zap" or "activate" has a chance to be affected. If the sanity effect comes into play, a random selection between the available options is made. Escape becomes more difficult if you confusedly read Remove Curse rather than Teleport. And if you're stockpiling poison for that gas trap, better be very careful before you reach for that ! of CCW.
Targetting - when each "target" is used by a command, be it zapping, firing a bow or giving targets to your pets, a random monster in LOS is used instead. You may have intended to noxious cloud that Cyclops, but your pet High Elven Ranger was LOOKING at you funny...
Panic - after any round in which you take damage, fear may result. Alternately, confusion.
Paranoia - On any round in which this takes effect, you dismiss 1d10 pets. They were SPYING on you, see?
Random Spell - Have a guess as to what happens with this one?
Just a few silly ideas...
2cworth and counting...
NeilStevens: You're cruel, you know that? And I don't say this in a bad way. I think if you had a free hand in ToME the game would be unwinnable. Maybe you should make a module, Excessively Evil ToME, or Challenge ToME, or something like that.
RavenRed: I'll take that as a... compliment... I, uh... think. And don't worry, there's quite a bit of stuff that is player-power-uppering in my vision for ToME as well. 
TheFalcon: I like these ideas even if they are a bit ... evil. However, I think maybe there should be something else, which could be positive. I would have thought insanity would make you less prone to running away: a GWOP comes round the corner, you don't teleport out of LOS, you cackle madly and charge at it. This could give you a chance to improve melee damage or chance to stun (though at the risk of getting killed obviously). Or maybe you could give resistance to wounds (think Monty Python: "it's only a flesh wound!!"). Hmm, anyway that's just a few more (slightly random) ideas.
BsVeteran: RavenRed, the idea is very in-theme with insanity. For balance, though, there needs to be a range of readily available methods for curing insanity (as with healing), without requiring a stockpile of !CxI occupying an inventory slot.
RavenRed: See the discussion in this thread for some of the issues in regards to this. But a re-jigging of ! of curing (+ Healing + CCW) rarities would probably help this significantly...
LordEstraven: This is great, insanity for real instead of those stupid hallucinations.
Of course, hallucinations should stay, they should just be "realistic"... Perhaps you're feeling paranoid, and that small fluffy dog that's following you around looks like a Zephyr hound? Perhaps it even breaths on you for bogus damage... Or maybe you think you're invincible, and that Great Hell Wyrm breathing down your neck looks like a fire lizard - perhaps you don't even notice when it blasts you with fire, to your great peril.
Which brings us to another thing - when you're insane, your health is going to be hard to keep track of! People who aren't in their right minds don't always notice when they're injured, and sometimes see wounds that aren't there.
And at high levels of insanity, the Sanity and SP meters should not me reliable ("Don't worry, man, I feel fine. Shut up, I'm perfectly sane!").
(Dang, this stuff would be *deadly* in a Cth module... >:D )