Dungeon Crawl Stone Soup

Dungeon Crawl Stone Soup is a roguelike dungeon crawler.
Available as pure ascii or with tile graphics, playable offline or as a web version.

DCSS let's the player create a hero from a large number of races and classes, then sends him off into the dungeon to fetch the Orb of Zot.
While descending through the dungeon, players will encounter a variety of different enemies to kill, sneak around, or even befriend.

The game features very in depth mechanics, comparatively simple controls and poses a very hard but fair challenge.