Before Halo, Marathon was the hot game Bungie Studios made. It was even for Mac! The series was naturally very popular in our little community. I fondly remember the afternoons I spent playing it. Rubicon X is a completely new game, downloadable for free.
This is really cool. There are all new levels, etc., and the whole thing runs on the Aleph 1 engine which is open source, so… you know… nerd points. I remember getting a demo of Marathon at Macworld Expo Boston in ’94. It definitely seemed ahead of its time, and it was great that it was for Mac.
Then Microsoft swallowed Bungie, and the rest is history.
Anyways, it’s free, so go get it.