Check out this self-balancing-on-a-single-ball robot that uses just two gyro sensors.  The original Ballbot was built at Carnegie Mellon University as an exploration of dynamically balancing robots with a narrow base, allowing them to travel in confined spaces.  Personally, I always felt the Ballbot was too large and cumbersome and not all that useful (it lacks arms, which would only cause balancing issues), and humanoids like the HRP-2 Promet have been shown to navigate very tight spaces by walking one foot in front of the other.  That said, this is a pretty cool hack:

Video (Mirror):

Aoki2001, its creator, has posted a number of videos featuring self-balancing robots of varying shapes and configurations.

[source: aoki2001 @ YouTube] via [ロボットのいる生活 (JP)]