These tiny brushed dc gear motors are just 12x10mm in cross section and similar to the popular Sanyo 12 mm gearmotors. They are available in ratios from 1:5 to 1:1000. Nominally running at 6V these motors will run fine up to about 9V but a shorter life can be expected as the voltage goes higher. You can get more life from the motor and suppress electromagnetic noise to boot by adding some capacitors across the motor leads.
Deceptively small, these motors pack a punch. Common ratios for wheeled robots are 30:1, 50:1, 75:1, 100:1 in either high or low current variety. Ratios either side tend to make a wheeled robot a bit slow or have a bit too low torque to get going fast. As a reference, the Pololu 3Pi runs 30:1 low current motors at 9V. See the video for a taste. The Pololu Zumo typically uses 75:1 high power motors at ~6V.
The output shafts are a D shaft shape making it easy to attach a wheel and lock it in place with a press fit or a locking screw. Power leads can be soldered easily to the tabs at the back of the motors. Specifications for speed and torque at given for the output shaft of the gearbox.