Check out Nexuiz. It is open source and quite a lot of fun!
http://www.alientrap.org/nexuiz/
Here is a video of gameplay:
http://www.youtube.com/watch?v=4E5kRPp9m3E
Not exactly free.
It requires bootcamp, but Tribes Next (a free version of tribes 2) is an awful lot of fun
Savage can be good too.. but i've had somewhat limited luck trying to run it under OS X