Twelve years in the making, the full version of Eternal, a 'near full conversion' for the open source version of the Marathon 2 engine, Aleph One, has been released:

Picking up from the end of the Marathon trilogy, you find yourself suddenly ninety-four years in the future, in the year 2905. You are on the S'pht moon K'lia, hanging in orbit over a desolate and ruined Earth.

Aleph One is available for Mac OS X, Windows, and Linux. Eternal comes in two editions; a Full edition (227Mb) and a "Lite" edition, more suitable for older computers.