Sandisk's Cruzer Micro is pretty darn small (not to be confused with its bigger brother, the Cruzer Mini).
Just FYI, I've read that some of the really small USB drives have problems when used with a non-powered USB port. You may have to get an external powered port to ensure consistent, trouble-free operation with some of them. I think some people have experienced this with the Cruzer Micro (but not the Mini, to my knowledge).