You will also find that if you take the installed files from a PC version of Quake 1, Quake 2 or Unreal Tournament and copy them to your Mac, and then download the Mac OS X update of the game, you can play these games on the Mac.
With the Quake titles, you simply copy the ID1 directories from the PC and download the OS X Quake from VersionTracker. (I'm not sure if this works for Quake3 as I don't own a copy).
With Unreal Tournament, you can simply copy the PC version over in its entirety, and then install the OS X update into the same directory.
Most games though are not as easy.
PS: If you have a legitimate use for them, emulators will run exactly the same files on Mac emulators as on PC emulators.