Nexuiz - open Source FPS
I may try this one out - Nexuiz, a free, open source first preson shooter (FPS) that uses the Quake3 engine. I've not played a game other than freecell in a long, long time.
Here's a review:
Nexuiz Hardware Requirements:
1 GHz Pentium III or Athlon,
512 MB of RAM,
GeForce 2 or equivalent,
400 MB disk space