Just ran into this very cool free game. It's a Q3 type online arena deathmatch game based on Q1's Darkplaces mod; GPL, free, nice graphics, windows and linux versions, nice servers to kick some butt in!
final note: turn all dynamic lights and shadows off to get good fps performance.
(not even high end cards will be fast here- a patch is expected)
http://www.jengajam.com/r/nexuiz
