Three classic MacOS games were released by Bungie for free. Marathon, Marathon 2, and Marathon Infinity.
The original MacOS Classic applications are available, as are just the data files, for those who want to use the open-source Aleph One engine. And that's cross-platform. Windows, Linux, and OS X.
Marathon Trilogy release -
http://trilogyrelease.bungie.org/
Aleph One -
http://source.bungie.org/
So go have fun.
Seems like a nice touch, following on the heels of GTA/GTA2 releases...