Alien Arena 2006

Support for Linux and Windows? A F-R-E-E first person shooter? You don’t say. And all thanks to id software releasing the source code a couple months back for Quake 3.
Cor Entertainment has released Alien Arena 2006 which has a major overhaul on the features list:

25 dazzling levels, DM, TDM, CTF, All Out Assault and Deathball game modes, as well as five new mutators, 9 player characters, 8 weapons, 3 alt-fires, and exciting powerups.

And the source code is released under GPL.
It’s no Quake 4, but it’s way better than Snake and Ladders.
Via Joystiq