Hm, the first one that comes to mind is clenching my jaw. If I'm concentrating on something or about to do something like.. I don't know.. Go to pat my dog. I realise I've clenched my jaw. I don't do this much anymore because I've realised I do it. I used to be really annoying.

Another one would be day dreaming. I could be here one minute and then up in another room the next and I can't even remember why I wanted to be in said room.

And buying video games. I guess it's not an unusual habit. But I have over 80 I haven't started. It's so bad. I feel a bit guilty. I need to get started. =].