Marathon: Aleph One is the name given to the open source effort to bring the Marathon game engine up-to-date with OpenGL 3D graphics. It runs on every major OS there is, and is freely available for download and contributions at source.bungie.org
After the Marathon series of games for the Mac OS proved to be a thorough success for Bungie, they decided to distribute their map-making and file-making tools with Marathon Infinity and also release the source code for the Marathon 2: Durandal engine (which was also used for Marathon Infinity with a few enhancements) under the GPL. Rather than just enhancing the old DooM-style engine any further, though, efforts began in 2000 to convert the entire engine to full 3D graphics, using OpenGL so it could easily be ported to other operating systems.
For copyright reasons, the Aleph One engine cannot be distributed with the actual maps, shapes, and sounds used in the original Marathon games. However, it's intended to be compatible with those files as well as any additional game scenarios fans might develop using Bungie's tools.
The use of "aleph one" is intended to indicate that this engine is one step higher than Marathon Infinity (since infinity in the usual numerical sense is aleph null in size).