This is such an easy one for me! I love it when @missy and @rambo pick good ones!
I have a handful of nicknames but the one most of my friends know me by is: Moo. This one was actually started by my sister (the estranged one) when we were younger. She would always call me Mary Moo, and her friends overheard her one day and it just stuck. Eventually it worked its way down to my friends and our cousins so for about the last 15 years or so, that's stuck. Why it's funny: I'm a vegetarian and super thin.
Another nickname that not very many people know (I'd venture to say only a few family members, if that) is Izzybee. This one was always used by my elderly Aunt Betty, who I adored. She died a few years ago and I miss her terribly. She loved all us kids but was especially fond of me since I was born. Why it's special: no one else has ever called me that and probably no one else ever will. Thinking of the nickname reminds me of all the great times we had and how special of a person she was.
And that's it!