Nexuiz GPL is a fun and free GPL deathmatch shooter released in 2005. It is a free game: source code is free software and data is free content.
The Full of Nexuiz version 2.5.2 release for Windows, Linux and Mac.
It also contains the Netradiant, a forked version of the GTK Radiant with some q3map2 fixes and total support for Nexuiz, Warsow, Quake2Worlds and the Darkplaces engine in general.
Of course also tools to run your own server and the intructions.