The Pololu Zumo for Arduino (click to view)

Zumo is the tradename for the Pololu mini-Sumo robot. A Mini-Sumo robot must be less than 10cm square and weigh less than 500 grams. Any height is permitted.

Pololu provide two main types of Zumo: one functioning as a "shield" for an Arduino microcontroller board, the second, a newer version with an integrated Arduino-compatible controller, which also includes encoders and a few other nice features.