I stumbled across this when looking for free FPS games... It's pretty much Quake 3 Arena (same engine) but re-skined and re-mapped to make it open source. You don't need to buy Quake 3 to run it! Just download the Linux tarball; uncompress it, download the Mac application (called MAC ENGINE), put the mac application in the linux folder you decompressed and voila! Because it runs on a relatively old engine, it runs great on old machines. Give it a try for a nice old-fashioned fragfest.