Adafruit Industries Adafruit Industries Mini Round Robot Chassis Kit - 2WD with DC Motors
This kit gives you everything you need to build the shell of a 2-wheel-drive Mobile Platform Robot! You get the metal plates that make up the chassis, two DC drive motors with matching wheels, and a caster ball for balance. You'll fill in the rest with a power supply, microcontroller board, and motor controller. The differential drive (two separately driven wheels) allows for a near zero turning radius while the high-strength aluminum alloy body plus high-quality high-speed motors make it suitable for flat indoor surfaces. Also it's adorably small!
Adafruit Industries Mini Round Robot Chassis Kit - 2WD with DC Motors