Update for me.
As I now have an (old) Windows-based machine in my room at college, there's no longer a need to use Parallels. Now, the games I play or can play on my Mac include:
Ceaser II (1996)
Castle Wolfenstein (1984)
Command and Conquer: Red Alert (1996) - for some reason I really like this.
Doom I (1993)
Doom II (1994)
Hexen (1996) - Doom in medieval times, far as I can tell...
Hexen II: Deathkings of the Dark Citadel (1996)
Quake I (1996) (plus Mission Park I: Scourge of Armageddon and Mission Pack II: Dissolution of Eternity, from 1997)
Sid Meier's Civilization (1991) - primitive!
Sid Meier's Colonization (1994)
Sid Meier's Pirates! Gold Edition (1993)
Sim City (1989)
Sim City 2000 (1993)
Stargunner (1996) - watch out, this one's addictive; love the music.
The Incredible Machine (1993)
The Incredible Machine II (1994)
Warcraft: Orcs and Humans (1994)
Warcraft II: Tides of Darkness (1995)
Wolfenstein 3D (1992) - my dad got me into this. It's on the Centris, too.
Wolfenstein 3D: Spear of Destiny (1992)
Worms Plus Reinforcements (1996) - rather oddly fun.
And that's just what I run through DOSbox emulators.
Still play OpenArena time to time, but the internet here sucks because so many people use it at any given moment, meaning I can almost never play online. :S
Also have Urban Terror, WarSoW, and Tremulous. All free and open-source. All a bit different from Quake III, on which they're based. Don't have Quake III though. It costs money. Not into that.
I play RuneScape sometimes, even. A bit boring, as I've played for nearly 6 years, and there are some compatibility issues with the latest version of Snow Leopard for some reason.
I have a .dmg port of Sid Meier's Civilization III for Mac (by MacSoft), but I think it's designed for Tiger. Lots of runtime issues on Leopard and Snow Leopard and I don't quite know why.
Long enough post. Happy time-killing, all.