A free, open-source pack of Bungie’s (of Halo fame) Marathon games has been released online for PC, Mac and Linux. Marathon began life in 1994 and ended in 1996.
The three game pack includes the newest edition of the Aleph One engine, an indepdent fan remake of the original game and HD versions of Marathon 2 and Marathon Infinity. It can be downloaded from SourceForge.
Multiplayer, co-op, control pad and OpenGL shader support is included.