"Wolfenstein: Enemy Territory (or ET for short) is a FPS (First
Person Shooter) game, native for Linux, which borrows concepts from
Return to Castle Wolfenstein, and uses the Quake 3 engine. The idea
initially was to develop it as a new commercial RtCW mod, but,
ultimately, it was released as a free standalone, multiplayer game,
after the developers abandoned the development of the single player
part of the game.
"It was released in May 2003, and ever since, it is one of the
most played online games..."