These tiny gearmotors are available in a range of gear ratios. All of the micro metal gearmotors have the same physical dimensions, so one version can be easily swapped for another if your design requirements change.
This combination deal contains most of the parts you need to build an Zumo Arduino robot. It consists of a Zumo Shield for Arduino, a Zumo chassis kit, and a Zumo blade. This programmable robot can be used as an autonomous robot.