Just before Bungie was acquired by Microsoft, they released the engine to
their acclaimed Marathon 2 under the GPL.  This has evolved into Aleph One,
which makes it possible to play the classic Marathon Trilogy on a wide
range of systems with improved features like OpenGL and internet play.

You will also want the marathon-trilogy package, which provides the
game data for the original trilogy. The marathon-scenarios package
(currently in wip) provides several third-party scenarios.

Also see:
   http://traxus.bungie.org  (information and more third-party scenarios)
   http://trilogyrelease.bungie.org  (the original free downloads)