My first "good" gaming computer was a Mac so I had ports of PC titles but also some Macintosh gems.
Marathon series: FPS with excellent storyline, great graphics (for it's time), and has gone opensource today.
Escape Velocity series: Something like a 2D Freelancer, this game was huge and had hundreds and hundreds of mods (then called "add-ons") for free. Third installment is out today and still has a huge fanbase.
Bolo: This sweet little tank game where you moved through different terrain and set up traps and defense while trying to destroy/outwit your enemy. Was awesome to play on Appletalk.
The Lost Mind of Dr. Brain: What can I say? It's an incredibly fun learning game. I play it from time to time just because it's replay value is still so high.