I have freakishly long limbs, fingers, and toes. On top of it, my second toe is as long as my pinkie finger and every joint on me is double. I can do the all the stuff Stimbo mentioned, and I can put both legs behind my head up to the knee joint. When I'm sitting, I tend to wrap one leg around the other, which my family has coined my "pretzel", and I'm most comfortable contorted into extremely compact positions, like right now, I'm sitting on my bed, typing with both knees under my chin. A friend once asked me to try to pick up a softball with the toes on one foot... that one took a bit of practice, but I was able to hold it for four seconds. I'm like a circus freak. I was also diagnosed with osteoarthritis at the age of 22, though it hasn't bugged me much at all since I moved here eleven years ago.
Nolite te bastardes carborundorum.