Supersonic Trebuchet
Cute.  It is around 8 feet long and per the comments, it throws tennis balls.  Throwing a tennis ball at supersonic speed is nothing to sneeze at, but the really impressive thing about historical and modern trebuchets is how big some of them got.  They could throw things like pianos, as in a famous Northern Exposure episode.  Doing that supersonically would have really been something ;).