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.

http://trilogyrelease.bungie.org  :  download the (now free) original games
http://traxus.jjaro.net  :  information and links to third-party scenarios