These small brushed DC gearmotors can deliver a lot of power for their size. This version has a 6V brushed DC motor combined with a 31.25:1 metal spur gearbox. The gearmotor is cylindrical with a diameter of 20 mm, and the D-shaped output shaft is 4 mm in diameter and extends 18 mm from the face plate of the gearbox.
Key specs at 6 V: 470 RPM and 170 mA with no load, 28 oz-in (2.0 kg-cm) and 2.9 A at stall.
These cylindrical brushed DC gearmotors are available in a wide range of gear ratios, from 25:1 up to 488:1, and with three different motors: 6 V and 12 V motors with long-life carbon brushes (CB), and a 6 V motor with shorter-life precious metal brushes. All three motors offer similar performances at their respective nominal voltages, just with the 12 V motor drawing approximately half the current of the 6 V motor and with the carbon brush versions having longer lifetimes than the one with precious metal brushes.
More technical information at Pololu.