Doom
Doom is split into three basic components, the engine, the Iwad file, and the optional Pwad file. The engine contains the code needed to run the game on a computer. The Iwad file contains the default levels, sounds, pictures, and other components. Pwad files contain custom levels. The source code to the Doom engine was released in 1997 and can be downloaded for free. The full 3-episode and 4-episode Iwad files have not been released for free and must still be purchased.