Jascha van Pommeren created the Unibot, a robot that pivots on a single wheel.

Video after the jump.

“The Unibot is a self balancing unicycle robot. This robot combines the principles of the famous wheeled inverted pendulum and inertia wheel pendulum. A well known application of the wheeled inverted pendulum is the Segway. Take one wheel off the Segway and add an other one perpendicular to and above the remaining wheel and the Unibot is born! The wheel on the ground is used to establish the stabilization in the y-z-plane. The wheel in the air is used to balance the robot in the x-z-plane. ”