The OmniTread robot was built by engineering students at the University of Michigan. Its body consists of seven segments connected by pneumatic bellows. Treads on all four sides of the segments give it traction against surfaces, and the connecting bellows can inflate or deflate to provide stiffness or flexibility as needed. The robot can squeeze through a four-inch hole or ascend a vertical tube.
http://www.engin.umich.edu/research/mrl/00MoRob_6.html via CrunchGear