Triple jointed outer ankles. Basically, I can fold a bone over them, and then fold another bone over them. I can make them crack rather loudly in doing so (if I want to), which has a tendency to nauseate people (yeah, I've had fun with that).
Oddly, I never knew that nobody else could do it until I was around eleven, when some girl sitting next to me in class was like "eww, your ankle's moving!"
I can also fold the finger bones to the sides of my knuckles on both hands, but only completely with the middle finger and ring finger. The index finger to a lesser extent.
.