Roguelike General Copypasta

Dungeon Crawl Stone Soup
Constantly being updated with new content, classes, races etc. A good introduction to the genre.
http://crawl.develz.org/wordpress/downloads

Nethack-One of the granddaddies of the genre.
http://www.nethack.org/

Elona-Detailed, huge RL with a JRPG theme.
http://homepage3.nifty.com/rfish/index_e.html

ADOM-Another classic with a giant world, runs in ASCII but don't let that discourage you.
http://www.adom.de/

Castlevania Roguelike-Castlevania lovingly made into an RL. Music, classes, monsters & more!
http://slashie.net/page.php?6

Rogue Survivor-Zombie themed RL where the goal is survival.
http://roguesurvivor.blogspot.com/

Dwarf Fortress-Besides the excellent mining & building main game there is an adventure mode.
http://www.bay12games.com/dwarves/

Red Rogue-Flash based platformer style of RL, still being dev'd, so new features always being added.
http://robotacid.com/flash/red_rogue/

IVAN-Iter Vehemens ad Necem
RL with a sense of dark-humor, I discovered here & love it.
http://ivan.sourceforge.net/

DOOM RL-It's DOOM! Guns, health kits, levers, more guns & a new graphical update so there's lot's & lot's of blood! All as a turn-based roguelike!
http://doom.chaosforge.org/

Brogue-Simple yet elegant, and very, very cruel. Familiar character based graphics are brought to a new level. You must play this.
http://sites.google.com/site/broguegame/

POWDER - A simple but fairly difficult roguelike in the vein of Nethack. Great for beginners to the genre, and also available on the NDS,GBA and PSP.
http://www.zincland.com/powder/index.php?pagename=about

Sil - A complex roguelike that boasts a very by-the-book Tolkien theme, with strong emphasis on tactical combat and discovery.
http://www.amirrorclear.net/flowers/game/sil/

Cataclysm- Post apocalyptic survival with detailed inventory and combat systems and surprisingly interesting/expansive crafting.
http://whalesdev.com/download.php