Descent !! free and ported to OS X. If you remember Descent 1, 2 and 3 from the olden days, you'll know what I'm talking about. Yeah! If not, it's a classic 3d shooter released not too long after the original Doom was released back in the early 90's.
True 3d, very playable, addictive
Now free and OS X native. Check it out at
www.applexnet.com. Look for an article on D2X-XL. _Be sure to read carefully and get the original game data too._
Others:
Doom Legacy - OS X native Ultimate Doom with hires textures
Noiz2a (or something like that) - a tripped-out Galaga
America's Army - free first-person shooter designed by the U.S. Army
By the way, here's a suggestion for searching for Mac games. In Google, type "free mac games" without the quotes. Also, you can generally play online games, Java, Flash, etc.
Doug