News of the existence of Halo 2600 broke over the weekend. I was surprised when I looked for it here on the wiki and couldn't find any mention of it at all. I don't know how strict you guys are about canon, but I think this is significant enough to at least have a short article for it. Here are some links to info:

The online Flash version

Download link for playing it on an emulator

Post by game's designer explaining how Halo 2600 came to be

Photo of some of the 500 real cartridges that were available at the Classic Gaming Expo

Gaming Bits news article

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