Bungie's first big hit, Classic Marathon, is free on Steam with Marathon 2 and Marathon Infinity to follow

Bungie's first real shooter hit, Marathon, has been released on Steam for modern hardware as Classic Marathon, a fan-led port that has Bungie's blessing to release. Classic Marathon is completely free, letting you have the rare experience of playing a classic game not only easily but with zero time spent hunting down an old copy and fighting with compatibility.

Marathon is a sci-fi FPS where your character is in charge of fighting alien forces that have boarded a human colony ship in the distant Tau Ceti system.