http://sauerbraten.org/I just started playing this game yesterday.
It's very nice for a free game. You run very fast and the guns are one hit one kill. And there's a chainsaw you can use to mow down enemies. And get this- when you die, there's physics in the body, so you can possibly die hanging from a ledge.
You can create your own maps.
You can even edit maps while players are playing on your server.
It's really fun, and free.
Check it out.
Availible on mac, linux, and windows