Aleph One is an open-source descendent of Bungie's Marathon 2 first person 3D shooting game. Aleph One supports Marathon 2 and Marathon Infinity data files (as well as other free third-party conversions). A conversion of Marathon 1 called M1A1 and many third party scenarios and net maps are also available.
Aleph One features software and OpenGL rendering, Internet play, Lua scripting, and much more.
It is available under the GNU General Public License (GPL).
(NB: Use of the hi-res textures, landscapes, and 32-bit Fusion Sprites are highly recommended.)