Nexuix is a fast paced 3D deathmatch game, developed and published online by a group of gamers calling themselves Alientrap. It's available for download for Windows, Mac and Linux.
Nexuiz is completely free to play, it's released using the GPL (link) - a first for a game of this kind. Nexuiz had its first release in May 2005, now after half a million downloads it has reached version 2.4. New features, maps and models are being regularly developed - check the forums.
For more screen shots and some game play videos have a look here (link), or for more information and a FAQ. You can ask the Nexuiz community here or chat on IRC (link to a page about Nexuiz IRC).
Nexuiz is completely free, the download is free, playing the game is free, the servers are free. Although everything about Nexuiz is completely free, the developers have to eat, please make a donation. You can donate with PayPal.