This is a mini robot that can balance even with a bunch of stuff on it.
Video explaining the challenges of balancing robots.
Hardcore project with a super big frame.
Less polished project but it's sill really nice.
This is a cool project. Not sure if it's still being sold, but they have some nice documentation. Wish I knew what motors they were using, though...
Here's another cool project. Goes a bit more in depth into the control theory aspect of the robot.