Aleph One (formerly known as the Marathon Open Source Project) is a free and open-source first-person shooter engine based on the source code of Bungie Studios' Marathon 2: Durandal.
The project commenced in early 2000, when Bungie released the code shortly before being acquired by Microsoft and spurred the fan community to further develop it. Since that time, Aleph One has become a successful project in terms of development and community support. Most modern-day and die-hard Marathon enthusiasts use Aleph One due to its enhancements over the original games and the fact that it is playable with today's platforms.
Initially, Aleph One was only compatible with Classic Mac OS (even though a version of Marathon 2 had been previously produced for Windows 95), but it has since been modified to work with SDL across a wide range of various platforms, including Mac OS X, Windows, Linux, BSD and BeOS.