Before there was a Halo, before there was Master Chief, there was Marathon: Bungie's first hit FPS, released in 1994. After Halo earned the company truckloads of money, Bungie released the whole Marathon Trilogy as freeware. As you might imagine, the graphics are a little outdated, but the gameplay and the storyline have aged well.
>>> Free Download Now - Marathon 1 (M1A1) - 37.7 mb
>>> Free Download Now - Marathon 2 - 25.9 mb
Choose a different mirror: http://trilogyrelease.bungie.org/