Wolfenstein: Enemy Territory (also referred to as Enemy Territory, ET or WET) is a free multiplayer FPS set during World War II. It 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).
The majority of Wolfenstein: Enemy Territory servers run various mods, such as Jaymod, No Quarter, ETpub, and ETpro.