http://sc2.sourceforge.net/
Thanks to these guys I can play this old PC game on my Mac OS X. I thought of you guys because what they do is similar in a lot of ways, they made a lot of things customizable (Sound and graphics) and they have this neat trick where you can download the music and voices, which comprise most of the data in the game, seperately as a zip file, and without unzipping it, you can put it in the "source:content" section and it works. They must have some program which unzips each file while the game is running.
There's also http://source.bungie.org/ which is where all the aleph one aka Marathon development is going on, but I don't think they're as cool as what you guys do.