All the time. I really wish I didn't though, since it's starting to affect my music. I can even remember when I started to develop the habit. I was in 6th grade, and there was this kid who would crack what, at the time, seemed like every joint in his body. So, I was in absolute awe, because, at age 10, cracking your knuckles is the most badass thing ever.
So I got him to show me how to crack my knuckles, and then I just...never really stopped. Eventually, it became a sort of nervous tick. I am currently capable of cracking my back, my neck (which I know is disgusting), my knuckles, my thumbs, and on occasion, my toes.