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