Sister Bear - My family nickname from my Dad. He's called me Sister Bear for as long as I can remember. I'm not sure if it's a Berenstain Bears reference, or what.
Auntie Mo - My nickname from my brother's two youngest kids. This actually started in 2001 when I worked at Michaels. My boss at the time, Harry, used to call me "Mo." When Austin started learning how to speak, he had trouble saying Milissa, so I said "He can just call me Auntie Mo!" and it stuck. :)
Mil - A former online friend decided to start calling me this one day, and it stuck. It's now my most common nickname. Don't call me Missy, or I might claw off your face... Unless you are my grandma, one of my Aunts, or my Mom.
There have been others through the years that have been used here or there. Like Missa...but the above three are the only ones that ever stuck with me.