MM is short for "Mega Milestone". Several of us around here like trying to get big milestones on as many projects as we can. For example, Mumps has 28 2.5M MMs, which means he has gotten at least 2.5 Million credits in 28 projects (which happens to put him #1 in the world in that category by 8).
Free-DC tracks them. It's a cool way to measure breadth of crunching.