Enemy Territory

Enemy Territory is a free multiplayer FPS set during WWII. It was originally planned to be released as a commercial expansion pack to the popular FPS Return to Castle Wolfenstein and later as a standalone game. However, due to problems with the single-player aspect, the multiplayer portion was released on May 29, 2003 as a freeware standalone game. In early 2004 the source code for the game logic (not the game engine) was released to the benefit of its modding community. The game uses a modified RTCW engine, itself being a modified Quake III: Team Arena engine (which has been open-source since 2005).

[Source: Wikipedia]