Ensemble Studios, the now closed developer studio, was apparently hard at work on a Halo MMO. If your mind isn't blown yet, it will be.
It's almost a shame that the game may never see the light of day now.
The title seems to have been in development in 2006 and some of 2007 — though it's by no means clear that it was the only MMO-related title in development at the studio at that time.
What is clear, however, is that Ensemble's Bruce Shelley mentioned in a June 2008 blog post that they "...set up three prototype teams out of the staff of a major project that we cancelled. After six months of very interesting work, we have now stopped two of those prototypes, with one getting more time to demonstrate the value of its concept."
Thus, it appears that the Halo MMO project was likely internally cancelled some time in mid-late 2007, without any formal announcement from Microsoft that it was ever in development. The below UI mockup and concept/level screenshots also date from the 2006-2007 period.
It's almost a shame that the game may never see the light of day now.