NetHack rules! By far the coolest way to play it is on a public server, where you have access to "bones piles", or levels where other players have died and dropped all their goodies. Kind of lends a multiplayer feel to it. Of course, usually the overwhelming joy at finding a pile of awesome equipment is tempered somewhat by the agony of getting whacked a couple of moves later by the monster(s) who did the first guy in.
If you can get used to the interface and (lack of) graphics, NetHack makes Diablo seem about as deep as Pong.
