id software released the Quake3 engine in 2005 under the terms of the General Public License. Even though the engine is eight years old, just recently two independent projects have released fully featured multiplayers games. All three games mentioned below use an improved Quake 3 engine, with OpenAL, Vorbis and SDL support, to run under Windows, Linux, and MacOSX.
Slashdot describes them as follows: “Urban Terror and World of Padman, formerly modifications that required you to have the original Quake III Arena game, can now be played independently as stand-alone versions. Urban Terror combines realistic environments and weaponry with movement similar to Quake3. World of Padman on the other hand is a colorful shooter in comic style giving you fun weapons like water balloons and water pistols to shoot with. Last but not least there is Tremulous, a first person shooter with added real time strategy elements which has been out for quite some time now.” Videos of all games below.
My favorite has to be World of Padman Standalone, see how crazy colorful the action is in the 2007 trailer:
Take a look at the awesome Tremulous promo:
Someone made a frags movie of Urban Terror: