Wolfenstein 3D
From Computer History Wiki
Wolfenstein 3D, written by the ever popular iD software, was a breakaway '3d' looking first person shooter game that defined the 3d shooter genre. Released May 5th 1992 It would take the world by storm. It was released as a shareware title which only included the first level. Users could then mailorder the rest of the game.
Ports
MS-DOS
The MS-DOS version was the best known
Open Source
The source code to wolf3d was relased in June 21, 1995., and was subsequently used in a 32bit port. The following site has information on building Wolf3d from it's original source.