Marathon was a networked first person sci-fi shooter game that Bungie released before the Myth games. They released 3 or 4 games in the series - many see a lot of similarities between Marathon and Halo. Like Myth it is a cross platform game, and has many user created maps/adventures.
A few years ago Bungie released the source code to Marathon and made it free - the user community has kept the game up to date adding better textures, and lots of other 'stuff' which I am sure someone who plays Marathon could tell you about.
Not sure this is the latest and greatest place to get the code, someone will undoubtably jump in if this isn't a good link - but the comprehensive version of the Marathon game engine (I think it combines elements from all 3-4 of the official Marathon games) is called 'Aleph One' and is available for free at http://source.bungie.org/.
Myth: 0
Myth II: 7
Myth III: 0
Marathon: 3
