Ed Fries (possibly of Mickey D's Fry Guys) went and made a playable version of Halo 2600, a throwback to what it would have been like to play Halo on the Atari 2600. Honestly, I couldn't tell the difference between this version and the one for the XBox, but maybe some of you harder-core gamers can pick out some minor nuances. Whatever the case, I made it to the second level before a zebra shot me in the face and I tore all the keys off my laptop in a fit of rage.
Halo 2600 [codemystics] (go here to play)
Thanks to John, Michael and T.J., who agree oldschool is the best school.