I guess this will show me as being old, but I played a beta of the original marathon when I was in college. It had just a couple levels and the alien weapon was way different than what it ended up as in the first game. I then spent so many hours of my college career playing deathmatches against my fraternity brothers. It was the most fun I ever had playing a video game.
I recently re-played the whole trilogy using Aleph One (I have marathon trilogy on a CD-rom) and I had forgotten a lot, and also, now that there are many websites dedicated to marathon, I found things I had missed on the first go-round. It's still a great game, and I'd pay through the nose if Bungie made another marathon game, or else made a Halo in which Durandal makes an appearance. They're no longer owned by MS, so why not revive Marathon? If they can make Duke Nukem Forever, then they can make Marathon 4: Durandal vs Tycho...
I am a huge Marathon fan. They were so far ahead of the curve at the time, and started on a Mac, my favorite computing platform. Marathon fanboy for life!
Oh, I forgot. You want to go to marathon.bungie.org for all things Marathon.