Mine is all 3 of my nicknames. Jibby - when my niece was two, she couldn't say "jeremy" so, it came out "jibby". And it stuck. Jack - I was the jack-in-the-box in the 6th grade play, so some of my friends call me jack. Joe - a chick in highschool didn't know my name, so she called me joe. I didn't know hers so I called her Sally.
Also, OP...fantastic idea for a thread. Refreshing not to read "OMG WOW IS DYING PANDALOLOLOL" every once in a while. +1 internets