256 has one - It's 27"x37"x40", weighs a little over 90 pounds, and just happens to score basketballs.

In all truth, our whole robot is our balancer; It has a few automatic balancing modes, a brake mode, which resets it to it's previous position in the event it's moved, and a few gyros to help it know it's position.

Surprisingly, robots can be great balancing mechanisms by themselves with a few sensors and some software.