Credit to IGN.com for this one – Microsoft’s Ensemble Studios had previously working on a Halo MMO in 2007. The project has been cancelled of course. According to IGN.com, there may be a chance that sometime in the future Microsoft may want to resurrect the project. Personally I think it would of been a flop anyway, seriously, can you imagine World of Warcraft with Masterchief? Nah, me either.
And now for your viewing pleasure, here are some screenshots of what could have been.