Lego of choice: The LegWay

http://www.teamhassenplug.org/robots/legway/

I’ve been able to put it on a table, and tilt the table, and LegWay continues to maintain its balance.

The Legway is a successful recreation of the famous, albeit a little expensive Segway, a transportation device which makes use of gyroscopes in order to provide stability with only two wheels. The Legway obtains the same results by adding proximity detectors (provided by hitechnic), to a mindstorms core. These sensors provide the core with precise and continuous measures of the distance to the ground, which are used to make the calculations that allow two motors to keep the thingy always up and proud.

The model shown is currently able to follow a line, and it can also be remotely controlled. Steve, the Legway designer, has also included links to the building instructions and the program he used, so you can try making one for yourself! (and if you succeed, next step is the real thing).