I am actually diagnosed with a form of OCD, where I have to be doing something with my hands at all times. Often they are harmless, other times they are... not, I'll just leave it at that. But this is a lighthearted thread, so here are some of the better ones:
I do finger drums all the time, were basically I play the beat of songs I know on any table I am at.
Constantly cracking knuckles, even after I just cracked them. Actually I crack just about every joint in my body.
Constantly readjusting my glasses, for literally no purpose.
Snapping, quite a bit.
Another completely unrelated one is I will mouth the words to most songs I like, with no prompting. This is actually why I stopped listening to music on the bus.