World of Warcraft is available on the Mac. The good thing is, if you have the PC version, you already have the Mac version as well.
WoW is one of those rare dual OS game boxes out there.
Another fav is the Halo series (btw, did you know that Halo was supposed to come out for the Mac originally until MS swooped in and bought the game developers before they can release it for the Mac? They had the developers do Halo for the Xbox as the killer launch title and the rest is history. Bungie eventually got around to releasing it for the Mac after a few years
Command & Conquer is great as well.
And now that Windows is running on Macs, just about any Windows game out there should run on the mini...
Be forewarned though, the Mac Mini only has an integrated video driver and not a full-blown graphics card.
You may want to look into the iMacs if you want to do any serious 3d gaming.