Nick Thatcher has designed and created his own self-balancing unicycle, with the help from an Arduino UNO. You can think of it as a cross between a Segway and a unicycle, but a lot cooler; it has a top speed of 10mph and a battery life between 1.5 and 2 hours.
The Raptor runs from a 24v 350W geared motor powered by 2 12v 7Ah battteries wired in series (to pump out 24v). An Arduino UNO takes all the data from the gyroscope, and sends PWM signals to the motor.
Nick has also made a Segway-clone, and is currently working on the Phoenix, an upgraded version of the Raptor.