This is the biggest of all the roguelike games. Descend to the deepest levels of Gehennom to retrieve the Amulet of Yendor, while acquiring weapons and magic and building your character's skills. I highly recommend the X11 version, which uses a custom font and better graphics. There is also a version which uses the Qt toolkit for X11, which is quite pretty. See the Qt NetHack web page for more details.

License: free