Did a search, was surprised to find there's no thread (unless I missed it, apologies if so).
So this is really cool. Ed Fries, late of Microsoft, got the blessing from his old employer to cook up this awesome piece of homebrew - Halo for the Atari 2600. He's at Classic Gaming Expo in Las Vegas this weekend selling a very limited edition of I think about 500 copies, sure to become a collectors item! I have already bagged mine :)
For the rest of you, a Flash emulation of the game is available at:
(site appears to be tempoarily down)
4K binary downloadable:
Facebook fan page: