Here's where to get it for the PC:
http://alientrap.org/nexuiz/
Here's where to get it for the Mac:
http://www.apple.com/downloads/macosx/g ... exuiz.html
It's no substitute for Halo, but it's actually pretty well done (graphically). The physics are a bit off and the gameplay is a bit fast, but it's quite fun.
This game supports both AI bots and online play. If you want to play me online sometime, my game rooms will always be "theycallmechad," and my game tag will always be "theycallmechad." See you there!
Screenshots:




