Return to Castle Wolfenstein: Enemy Territory
Enemy Territory began life as an expansion pack for 2001's Return to Castle Wolfenstein. Activision ended up canceling the game, but it released the multiplayer component of the game as a free stand-alone download in 2003. The result was a game that--years later--holds its own against such heavyweights as America's Army (above), Call of Duty, and Unreal Tournament 2004 as one of the most played online multiplayer shooters out there. The game's legacy will live on in the form of Enemy Territory: Quake Wars, a multiplayer spin-off of Quake 4.

Download complete Windows game (264MB) from CNET Download.com: Free download
Download complete Mac game (271MB) from 3dgamers.com: Free download
Download complete Linux game (271MB) from 3dgamers.com: Free download