Quake II
Quake II is a first person shooter game developed by Id Software and distributed by Activision in 1997. It is the sequel of Quake.
Unlike Quake, where hardware accelerated graphics controllers were supported only with later patches, Quake II came with OpenGL support out of the box.
Quake II uses a similar client/server network model as Quake.
The server code of Quake II, which defines how everything behaves in the game, can be changed in any way because Id published the source code of their own implementation and Quake II uses the shared library functionality of the operating system to load the server program at run-time.
The source code to Quake II's client program was released under the terms of the GPL on December 21, 2001.