Remote controlled sphere
This is a remote controlled sphere, not unlike the sphero and several DIY variants. It served as a great opportunity to test out inertial sensors & RF control, as well as being a birthday present for my nephew.
The sphere mechanism uses a gear motor and a servo. This seems to be the most popular method in DIY implementations. The alternative is the "car in a sphere" arrangement as envisioned by (among many, I'm sure) xkcd, and implemented by Sphero.