120:1 Mini Plastic Gearmotor HP, Offset 3mm D-Shaft Output
This compact, inexpensive plastic gearmotor (high-power brushed DC motor with a 120:1 reduction gearbox) is well suited for use in small robots. While intended for operation at 4.5 V, it should run comfortably in the 3 V to 6 V range. The gearbox has a 9.7mm-long, 3 mm-diameter D-shaped output shaft that is offset from but parallel to the motor shaft, and a built-in safety clutch helps protect the gears from excessive loads.
Key specs at 4.5 V: 150 RPM and 130 mA free-run, 25 oz-in (1.8 kg-cm) and 1.25 A stall.