I'm playing Dungeon Crawl Stone Soup.
Theres an ascii version with online functionality, but I'm playing the graphical tiled version. Its pretty awesome and free.
What makes it so playable for me is the auto-explore and auto-pickup features. You just hit o and you run until u see something. Makes it really fast to clear dungeons after you just died and groan at starting again.
Also, loads of sweet races and classes. Stuff like minotaurs who make passive horn attacks, kenku flying bird people, centaurs, draconians, vampires... And if you prefer more classical races theres 3 kinds of elf, 2 kinds of dwarf, etc, etc 
My favourite roll is probably Orc Priest of Beogh. As you gain piety through killing NPC orcs have a chance to think you're the orc messiah and permanently join you. Then they'll pick up items and level up as they travel with you. And you can get heaps of them! 
You also have a damn nice Smite spell, which is a god power, so you dont need any casting ability.
Another fantastic roll is a robe wearing Merfolk Crusader. That roll has my top score so far.
Also, kenku birdzerker was a laugh. Tremble before my mighty beak! 
EDIT: linky http://crawl.develz.org/wordpress/downloads