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 156.3:1 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: 93 RPM and 170 mA with no load, 110 oz-in (7.9 kg-cm) and 2.9 A at stall. Note: the gearbox torque limit is 70 oz-in (5 kg-cm); see detailed description for important product limitations.
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.
Detailed information at Pololu.