I am absolutely addicted to cracking that I crack in my sleep. In my sleep, I usually crack my neck, my back and my fingers. I do it so often sometimes I wake my husband by accident and he yells at me. He hates that I do it and swears I'll regret it one day. I can't help it though. It feels so good and I love the sound of it. My favorite is cracking my neck; though it used to be my back but its been a long time since I've gotten a really good back crack.
I often have fantasies of being strapped to a device and having x-ray videos taken specifically of spots I crack; it would be cool to actually see stuff happening while I crack. But I doubt highly that such a machine exists so I will continue to crack on my own and be satisfied that way.