[Duke]

It's not the premiere gaming platform, but the mac has had its fair share of very fun games. The first couple found their way to the PC as well (Dark Forces was a simultaneous release, AFAIK), but I consider them Mac games because I never heard of anyone playing them on a PC.

Dark Forces

Basically Quake, but in the star wars universe. It wasn't "truly" 3-d, but you could aim up/down. Boba Fett was damn tough to beat.

Marathon

An excellent (for its time) FPS from Bungie. You may have heard of the company, but before '96 the game was mac-only.

Apeiron

The best Centipede clone I've ever played, from Ambrosia Software.

Tetris Max

IMO, this was the best version of "standard" Tetris ever created. It was a mac only, and is probably impossible to find these days. I'd pay $10 or whatever for an OS X build of it, though.

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Let's add to the list, and if there are any current gems out there fill us in!