I love Nexuiz , unfortnaly hardly anyone in australia plays it and its too hard for the noob gamers to play and they revert back to BF2/CS/CSS/COD/DOTA mod/WOW
Abuse SDL is a lunux and Mac free game - http://www.labyrinth.net.au/~trandor/abuse/
Wolf ET is run , but its now too popular with the noob players and they map pack and mod it all up.
Quake 1 and 2 are pretty small and on Linux ...
UT 99 GOTY is a great FUN game and works in Windows and Linux