Hey guys, try out Xonotic!
It is the successor to Nexuiz, which is another open-source fps that is dead and was re-made into a 'full' game you have to pay for.
Imo, Xonotic has really good gameplay and incredible graphics, all for absolutely nothing. There is also an active community and the game is always changing.
Version 0.6 is out NOW, so get fragging.