Quite possibly the simplest design for a robot that could (sort of) balance itself on two wheels, without the use of accelerometer, gyroscope or microcontroller.
A great weekend project, and do check out the video to see it in action. And why Domo Kun? Because my other half think it's cute :) You can pretty much change the WobblyBot into any character you like.
UPDATE: The WobblyBot was featured in Hacked Gadget, Engadget and Hack-A-Day!
How It Works
It's about simplifying things and getting back to basics. the goal is to build a balanced robot instead of a robot that balance itself.
The robot is essentially a simple pendulum, with the pivot at the wheel axle. The bottom part of the robot’s body is significantly heavier than the upper part of it. This serves as a counter weight, keeping the entire body upright, hence the balancing act.