I crack my neck all the damn time, its really bad haha. I've never been a fan of biting nails, I guess I saw my friends do it when I was younger and was grossed out by it.

Another thing I do is.. I'm not even sure how to explain it, but here goes. I guess I tweak my lips around subconsciously, if that's what you want to call it. If I'm bored sitting in class or whatever, I'll be moving my mouth and lips every few seconds, it's hard to discribe lol. Dunno why I do it, just kind of a stress reliever. I also shake my knee a lot in class if I'm working on an assignment or test or something too.