I can crack my knuckles (it's a myth that it causes arthritis - it's only a symptom, not a cause). I can do this weird thing where I displace the bone on top of the knuckle on my middle finger on both hands. The visible bone that runs from the wrist up to the finger moves out to the left or right of the knuckle on the corresponding hand. I do loads more bones but those are the two that really annoy and psyche out all my friends.

I drum my fingernails on a surface when I'm bored or annoyed. When I think a phrase is witty I repeat it incessantly until it is no longer funny (ie: "No , Sherlock" or "gaspola")