The Pololu Zumo 32U4 robot is a versatile tracked robot based on the Arduino-compatible ATmega32U4 MCU. It includes two 100:1 HP micro metal gearmotors along with integrated dual motor drivers, an LCD, quadrature encoders, line sensors, side and front proximity sensors for detecting objects, and a full IMU for detecting impacts and tracking orientation. The low-profile robot is less than 10 cm × 10 cm—small enough to qualify for Mini Sumo. No soldering or assembly is required; just add 4 AA batteries and a USB cable and your Zumo is ready for programming.
The Zumo 32U4 is a highly integrated, user-programmable and customizable tracked robot. It measures less than 10 cm on each side and weighs approximately 275 g with batteries (170 g without), so it is both small enough and light enough to qualify for Mini-Sumo competitions, but its versatility makes it capable of much more than just robot sumo battles.
More technical information at Pololu.