Dungeons of Dredmor. It's a softcore humorous roguelike (with graphics!), so probably nothing for purists, but it's pretty damn good.

Cardinal Quest and Hack Slash Loot are both interesting "streamlined" takes on the RL genre, and are easily worth checking out. Desktop Dungeons is good too, if you don't mind the constantly-updated beta currently being a webapp.

Spelunky? Roguelike meets platformer, maybe not exactly what you where looking for, but very good (and free)

NetHack seems the obvious one

Look here - http://www.gog.com/en/forum/general/roguelikes/page1 or here - http://www.gog.com/en/forum/general/looking_for_more_learn_as_you_die_games_roguelikes_goblin_camp_etc

Or to numerate my favourites:

ADOM - http://www.adom.de/
Dungeon Crawl stone soup - http://crawl.develz.org/wordpress/
Infra Arcana - http://infraarcana.wikispaces.com/
Brogue - http://sites.google.com/site/broguegame/
DOOMRL - http://doom.chaosforge.org/

+ Dungeons of Dredmor (steam)
Spelunky - http://spelunkyworld.com/

I don't think you will ever need another game.

Don't forget and its [url=http://roguebasin.roguelikedevelopment.org/index.php/List_of_Angband_variants]many variants.

would also recommend ADOM

Telengard. Get it while you can. There may still be a working link somewhere in this thread: http://www.gog.com/en/forum/general/save_telengard/page1

ADOM - a hard, supremely rewarding experience. It's not just a randomly generated dungeon, but a whole persistent world of towns and dungeons. Currently on my eighth playthrough. Here I come, Andor, you mangy bastard!

DoomRL - a light kind of roguelike. A shooter in spirit, but with enough depth and complex mechanics to make it interesting and friggin' awesome.