I built a BB-8 robot, as seen in the new Star Wars trilogy. This was the second robot I built, and I started it right after the trailer for episode 7 was released. It moves by tilting a large mass and compensating for the horizontal deflection with motorized wheels that run along the interior of the ball. There is also a servo to turn the head, which is mounted on top of the sphere with powerful magnets.
My budget as a broke college student was under $200, so the actual ball is a lamp globe. Because of this, the robot had to be assembled like a ship-in-a-bottle in 3 separate pieces.
I used to be really bad at documenting my projects. I still am, but I used to, too. So the only videos I have are very low-res testing videos: