Aleph One :: Marathon Open Source Project
Aleph One is an open-source version of Bungie's amazing Marathon engine for Mac OS X, Linux, and Windows. Since Marathon the game is no longer for-sale, Bungie has released the engine to the open-source community, but (unlike other companies like id Software that have done similarly) they have also released the full set of assets (graphics, sounds, maps, etc.) for all three Marathon games (Marathon, Marathon 2, Marathon Infinity). This means you can download and play the game today! I spent many, many, hours playing Marathon on my old Power Mac 7200/75 in college, back when it was pretty much the cutting-edge FPS. Even today, the trilogy stands out for its incredibly-detailed story line and mythology and its vast library of user-created content. Very highly-recommended.