I don't crack my knuckles... It's the fact that I'm either always playing guitar or using one of those hand grip exerciser things.
Yeah, I used to fish like hell for anything form striped bass to sharks on top of it but then I quit when I became a vegetarian... then I used to play baseball, but quit because I really just did that because Josh was doing it and on top of that I always use hand exercisers and I always have a guitar within reach and I play constantly when I'm at home and often take guitars with me in the car on trips or when I spend the night at peoples houses... I even keep a Martin Backpacker and a Fender Telecaster at my grandmother's house, so if it's not arthritis, I really can't say it's anything else.
Also, one of my legs is shorter than the other... of course, they usually are, but mine is to the point where if I put a magazine under my right leg, it feels even... plus I have a really oddly crooked big toe on my right foot... though, they say it doesnt effect anything. Goes way over to the right... my grandparents first noticed it when I was about two.