Aleph One is an open source project based on the source code for Marathon 2: Durandal.
The Aleph One project started with the source code for Marathon 2: Durandal, as released by Bungie Studios, and has continued to develop the code to maintain compatibility with current platforms as well as expanding to platforms that were not originally able to play the Marathon Trilogy. As of 2012, the project is still under active development. The project is covered under the GPL (GNU General Public License).
Either directly or through additional conversions, Aleph One allows for all three games of the Marathon Trilogy, as well as third-party scenarios, to be played on Mac OS X, Windows, and Linux. Aleph One also serves as the basis for the iOS releases of the Marathon Trilogy.