A roguelike is a game in the genre of ASCII RPGs, the first of which was Rogue in the 1980's. While the mainstream gaming industry has gone down the path of powerful graphics, Roguelikes continue to thrive due to their rich depth of player options, wide range of settings, addictive gameplay and imaginative storytelling.
The one, the only, the original Rogue.
One of the classics, and still one of the most popular roguelikes.
My personal favorite roguelike, with Tolkien-esque creatures and artifacts.
Fuck yes Castle of the Winds you are the best Windows roguelike ever
Yes, it's seriously Doom as a roguelike. It's also actually a lot of fun; it uses all the Doom sound effects and music, and uses them well (including directional environmental sound effects and monsters breathing).
Direct download with tileset
Mac OSX version
/tg/'s current roguelike of choice. Fucking Sigmund.
Dwarf Fortress Wiki
May Green graphic set
Kobold Camp - /tg/ original content!
Incredibly complex game where you build a fortress in a mountainside and defend it from invaders.
A very pretty, and pretty fucking weird, roguelike. One of the best games graphically on this list.
LIVAN (Expanded version with tons more stuff)
A roguelike comparable to Dungeon CRAWL, but much more difficult.
A roguelike that uses the Dungeons and Dragons 3.5 Edition ruleset.
One of the first and still one of the best.
A zombie apocalypse roguelike. Pretty awesome.
A roguelike remake of Space Hulk, a Warhammer 40k-themed boardgame.
"Dark science fantasy", Warhammer 40k-themed roguelike.