http://www.slashgear.com/hoverboard-is-a-self-balancing-one-wheel-skateboard-22405876/ Then there's the cheaper "one wheel" for 1500 clams. https://www.youtube.com/watch?v=uqt-wn6C_fg
I watched a drunk brazillian girl flop flat on her back trying to ride one of these things in PB Last weekend. Not gonna lie, It made my day.