AlephOne

Aleph One is an Open Source 3D first-person shooter game, based on the game Marathon 2 by Bungie Software.

Aleph One supports Marathon 2 and Marathon Infinity natively, and Marathon through a conversion called M1A1 and 3rd-party content on a wide array of platforms, with (optional) OpenGL rendering, Internet play, Lua scripting, and more.. Many third party scenarios and net maps are also available.