Forum user Erich created a custom Zumo Robot Chassis PCB to use with our Zumo chassis kit. The board is designed to accept a large number of plug-in modules, such as a DRV8835 dual motor driver carrier, encoders, and voltage regulators. Sensors that can be mounted on the robot include a Zumo reflectance sensor array, some distance sensors, and an ultrasonic sensor, and it also supports several wireless communication modules. Instead of an Arduino, it uses a Freescale FRDM-KL25Z as the microcontroller board.
You can follow his robot’s progression by visiting these forum posts:
March 2013: Zumo Robot with FRDM-KL25Z Board
September 2013: Zumo Robot with Pololu Plug-in Modules
October 2013: Zumo Robot with Pololu Plug-in Modules, assembled.