NetHack

NetHack is a rogue-like game. It is licensed under the NetHack General Public License.

Gameplay
The player moves in turns in a dungeon, finding items and killing monsters to retrieve the fabled amulet of Yendor.

Nethack has a high replay value as every game is unique; essentially every level is randomly generated, items and treasure are randomly placed with unique and functional descriptions, and monsters are generated continually and randomly. There are thirteen different roles, 5 different races, and 3 different alignments. Nethack is extremely difficult, nearly impossible. The vast majority of its players have never beaten the game.

History
See NetHack release history for more details.

Variants
The most popular variant of Nethack is SLASH'EM (Super Lotsa Added Stuff Hack - Extended Magic). It features many new monsters, roles, races, items, levels, etc. It is also considered much more difficult than the original Nethack.

Another popular variant is the graphical Falcon's Eye. It is mostly the same as the original, except it uses isometric graphics, instead of ASCII. It has since been forked as Vulture's Eye.