Originally made by ID Software, Quake is now opensource.
There are many versions of the mac quake engine available (eg, NPR Quake, Atari Quake, Fruitz of Dojo Quake...etc), so I have linked to the most popular one GLQuake .
For those who have been living on another planet ( :D mars?), Quake is a First Person Shooter similar to other classics such as the Doom series, Duke Nukem and Wolfenstein.
Battle through the official levels battling monsters and demons from hell, and then download custom made maps from other players out there and try new, unseen maps and levels....the opensource aspect makes the variability virtually endless.
A True classic and one of the finest FPS' out there (think lots of Blood, Guts and Gore).
Intel macs have varied success but just try different engines to find what works for you.