The original Mac only version of the game and its manual are available to download for free at http://trilogyrelease.bungie.org/. Individual files can also be downloaded as opposed to the full game. The game has also been modified to no longer require serial keys to install.
Note that they are provided as is and will only run on a classic Macintosh. The game is also using an older version of the Marathon engine, so features provided in the sequels such as mod support are unavailable.
It is recommended to use the Aleph One version as it adds modern PC support and a port to the newer engine.
Aleph One is an open source continuation of the Marathon 2 engine. Along with bringing all the benefits of the newer engine to the original Marathon, it makes the game more modern and accessible with many different improvements such as cross-platform support, widescreen resolutions, and improved multiplayer networking.
For Windows and OS X, everything is provided as a single app/folder. For Linux, the engine need to be downloaded separately from the game files, compiled, and merged in.
Both the engine and the game files can be downloaded at https://alephone.lhowon.org/.
As a bonus, the game files provided by Aleph One include HD textures, a revamped HUD, and a new UI.
A legal serial key generator for the retail versions of all Marathon games. Refresh the page to get a new batch of keys. Marathon keys are stated with "M1" before the codes.
It can be found at http://marathon.bungie.org/maraserialgen/.
In-game general OpenGL settings.
In-game advanced OpenGL settings.
<marathon><view><fov normal="120" extra="160" tunnel="70" /></view></marathon>
In-game key map settings.
In-game mouse settings.
In-game joystick settings.
Aleph One plugins that change the in-game HUD to match the original release. There are two variants available: