(YouTube Link)
The Galileo Robot has retractable wheels within its rear wheels that extend on command, expanding the hub of the wheel into a tank track. This allows the vehicle to have the advantages of a tracked vehicle when off-road, but the advantages of a wheeled vehicle when on a smooth surface. One application that the developer, Galileo Mobility Instruments, has already developed is a wheelchair that allows users to climb and descend stairs.
Company Website via Make Magazine
http://tubedubber.com/#luX_0Ek8cqc:ZnHmskwqCCQ:0:100:0:0:true