Xonotic (Zoh-nah-tick) is a Quake 1 derivative, based off of the Darkplaces engine.
It's fast paced, competitive, and boasts as good a community as our own! It's open source for all of you obscure OS users, so you can compile it yourself if necessary. The download comes with a 32 and 64 bit windows version, a mac version, and a linux version.
But best of all... IT'S FREE! (AYYYYYYY!)
I suggest you snag a copy, so I can be mercilessly fragged by more than just the regulars.
http://www.xonotic.org/ (http://www.xonotic.org/)
In terms of system requirements, here's a collective list of people who posted their specs and results.
http://dev.xonotic.org/projects/3/wiki/Hardware_Requirements (http://dev.xonotic.org/projects/3/wiki/Hardware_Requirements)
See you on the field!
It's a "new" game, but I wonder why they took off Nexuiz Classic and had this instead.