"First-person shooters (FPS) are probably the most controversial
games that exist. For some they represent the essence of video
gaming, others despise the ubiquitous violence or simply find them
extremely boring. Nevertheless, the genre has played an important
role in computer gaming history. Therefore it is logical that quite
a few open source FPS games have been created.
"In some cases one has to differentiate between the game engine
and the actual game content (graphics, maps, sounds). Several
engines of the Doom and Quake series have been put under the GPL by
id Software, and they continue to be the building basis on top
which many of today's open source shooters are built..."