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. Its name is taken from the second infinite cardinal number in transfinite arithmetic and references Marathon Infinity, the final game in the Marathon Trilogy.
It's a miracle that Bungie have allowed their spectacular Marathon trilogy and the complete toolbox to be released completely for free! Marathon 2 is my all-time favorite video game, and someday it'll be a honor for me to develop a mod or two for the series. :)