A fellow on the SuSE forums gave me this heads up:

"Marathon Games Now Available for Free"

"Marathon, Marathon 2, and Marathon Infinity are now all available free of charge. Marathon 2 and Marathon Infinity are fully compatible with Aleph One; just replace the Marathon 2 or Infinity application with the Aleph One application, and enjoy the benefits of OpenGL graphics, improved networking, and everything else that has been added to the game engine since it went open-source in 2000.

You'll find the Marathon games for free at http://trilogyrelease.bungie.org/"

The Marathon series were unique and interesting first person shooters with a rich story and puzzles that made you think. I recommend them to anyone who hasn't tried them, or anyone who has in the past and wants a little nostalgia.