prboom is an open-source port of Doom, the classic 3D first-person shooter
game. It totally outclassed any 3D world games that preceded it, with amazing
speed, flexibility, and outstanding gameplay. The specs to the game were
released, and thousands of extra levels were written by fans of the game; even
today new levels are written for Doom faster then any one person could play
See also: prboom-plus.