I've been searching around Wikipedia, and found a group of people have added mini-articles for the various scenarios out there. I would love to see a nice archive of Marathon information on the wiki, so I've edited Rubicon, and added Portal of Sigma and RED, however, I can't do this all by myself. Add your bit of info to the articles, and we'll get a nice result. =D
http://en.wikipedia.org/wiki/Marathon_%28c..._game_series%29