Nexuiz - a free opensource FPS game

SVG logo of the FOSS game NexuizNexuiz is a fast paced 3d deathmatch game(FPS). It's open-source, you can play it on windows, linux and mac freely.

Nexuiz is primarily multiplayer (though it includes a full single-player campaign, which allows one to play through the various multiplayer game types and maps with bots), and allows for hosting and joining of games. It can also support new gametypes, or whole conversions quickly applied to it (much like Quake). Nexuiz supports most Quake modifications (although with varying functionality). Currently there are a few custom mods and maps used.(via wikipedia
The latest version is 2.5.2, it includes 33 official maps(of course, you can download hundreds of community created maps), you can play many mods with bots or join a online game easily.
Nexuiz has a very active development team, and it's very open, you can contact them by email, or their IRC channel #alientrap on irc.anynet.org, if you would like to help with development. New release can be seen every year, and you can see many new Features like maps, weapons, etc.

See System Requirements and download visit its official site

2010.10.26 - Update :
Nexuiz founder have licensed the Nexuiz name, Nexuiz.com domain, and DarkPlaces engine to Illfonic in a deal to get Nexuiz on consoles. And the kink is that the engine has been licensed for non-GPL usage. That is, Illfonic has no intention of contributing their code back to the main GPL Nexuiz project. As a result, Nexuiz has been forked into a new project called Xonotic. While the main Nexuiz site doesn't mention that Illfonic has no intention of contributing back, the Xonotic project FAQ explains what's going on. Additionally, the Xonotic project states that Illfonic 'may be in violation of the GPL as most contributors to the Nexuiz codebase have not relicensed their work for inclusion in a closed-source project.'
(news from Slashdot)

The new Nexuiz by Illfonic: http://nexuiz.com/ 
The Xonotic Project: http://www.xonotic.org/