Wolfenstein: Enemy Territory

Wolfenstein: Enemy Territory (also referred to as simply Enemy Territory, ET, or W:ET) is a free and open source multiplayer first-person shooter video game set during World War II. It was originally planned to be released as a commercial expansion pack to the popular FPS Return to Castle Wolfenstein (also referred to as RTCW) and later as a standalone game. However, due to problems with the single-player aspect, the multiplayer portion was released on 29 May 2003 as a freeware standalone game. In January 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 Return to Castle Wolfenstein engine, itself being a heavily modified Quake III: Team Arena engine (id Tech 3, which has been open-source since 2005). As of the first day of the 2010 QuakeCon, 12 August the entire source code was released under the GNU General Public License v3.