I love the Marathon series.
Back in the day, before we had the net or anything, the computers at my house were all Macs. My dad's a digital retoucher so it was always Mac back then for photoshop, thus I came to play Marathon -  a Mac exclusive at the time. Its for this reason that alot of people missed out on Marathon, and found Doom instead, since it wasnt ported to pc till much later. For those that still enjoy older games though, it is an absolute gem, and has spawned an insanely devoted cult following..
Is anyone here perhaps among the ranks of the Marathon geeks?
For those that havent played the game, there are three Marathon titles, follwing on from the first, where you start on an enormous ship converted from one of Mars' moons. Aliens have rocked up and are fucking shit up. This is first contact. AI's have come a long way, and a perculiar ability called "rampancy" has been noted in some AI's if certain conditions are met. The rampant computer becomes self aware and starts to gobble up information and fling thier 'arms' about like a newborn.. of course the time frame a computer to learn an understand shit is a little quicker to say the least. So here you are inside this space behemoth, with aliens fucking shit up and dicking around with your computers, who's behaviour makes 2001's Hal look like Mary Poppins..
The game was made by bungie, way before they decided to read Larry Nivens "Ringworld" novel, and pretty much copy it into a game. The ship is dark, the tension is thick, and the exclaimations of "FUCK!" as aliens babble gobbledygook and run at you out of shit know where are about as geniune as it gets.
Better still, it is all available completely free. All you need is the program that reads the Marathon game files, and the files themselves which can all be found right here. Obviously you'll get the most out of it playing linearly.
http://source.bungie.org/index.php/Main_Page
Somewhere in the heavens... they are waiting.