I developed a small 2 wheel drive robot. It is Arduino based with most of the mechanical and electronic parts coming from the polulu website. There is a small gripper in front that is convenient to perform small tasks with the robot.
I did most of the code on-board including the communication protocol, the encoder management, the speed and position control, LED control, gripper control. The communication with the computer is via Xbee, there is a Python library to control the robot. Finally, there are some Matlab tools to simulate the robot and make it follow a virtual line.