Games For Mac

Left 4 Dead 2 works for mac. I'd strongly recommend it. No need to get l4d 1 considering the second one has the same campaign plus a lot more.

witcher?

The Binding of Isaac!!!

Going off my library right now, all HIB5 games, Torchlight (and Torchlight 2 once it's released), Guild Wars 2 (Mac version still in beta but works), Deponia, Portal 1/2, The Witcher (no idea about sequel, think that's Windows only). All games by Amanita (of Machinarium fame) have Mac ports.

I can't attest to OS9 emulation (or earlier) for the older games since I haven't tried running OS9 compatibility since 10.3 OSX or earlier.

dungeons of dredmor

The Walking Dead and Rochard are my favorite ones, so far. I also enjoyed Dustforce, Super Meat Boy and both Shadowgrounds-Titles. If it's not a too huge kick into the nuts for you, Duke Nukem Forever is also quite fun on the Mac. And sadly, Batman Arkham Asylum and Arkham City aren't SteamPlay-Titles, but the Mac Versions of these games you get via the Mac App Store are pretty good ports so far. And if you want something fricking hard to play, get Velvet Assassin for about 3 bucks. A WWII-Stealth Game with a female lead character.

edit: Oh and I forgot Jamestown. A topdown space-like shooter done so cute, that I even got a female friend interested in playing it.

CRITTER CRUNCH.