Aleph One is the open source version of Bungie's Marathon game.

Marathon is a three-part, first-person shooter series that was published by
Bungie Software. The complete Marathon trilogy consists of "Marathon",
"Marathon 2: Durandal", and "Marathon Infinity". The hallmark of the series
is the detailed story-line that unfolds throughout the game.

After Bungie released the source code to the Mac OS version of Marathon, the
Aleph One project was created to enhance the software and port it to other
operating systems. As AlephOne has matured, so have the user-contributed
scenarios, some which far surpass the original Marathon game scenarios
themselves.

Aleph One is OpenGL-accelerated, but performs well without OpenGL if you do
not have a graphics card with the appropriate functionality (you need to use
the -g/--nogl option).

WWW: http://source.bungie.org/