One of the last
things Bungie did before Microsoft bought them (and their flagship game, Halo), was to release the Marathon code into
the open source arena. Until not all that long ago, in order to play Marathon 2 or Infinity, you had to run it on a Mac
OS 9 machine, or in Classic (or, shudder, on a Windows machine).
Enter Aleph One, the end result of some programmers having ported the game(s) to run natively under Mac OS X. They've included also the ability to connect to an Internet game server so you can play against other Marathon addicts around the world.
It's like it's 1998 all over again. I'm going to have fun with this over the long weekend. Who needs World of Warcraft when you've got Marathon Infinity...
Update: This is a better link for a download that includes the necessary game files as well as the carbonized application. The previous link was for just the application by itself.