These Planetary Gearboxes are nominally 32mm in diameter with permanent magnet reversible DC motors. We carry 12VDC and 24VDC motors and range from 8 to 265 RPM. Several right angle gearboxes. Several motors have encoders.
The Dual Motor Shield is quite a practical Shield for Arduino with a library developed by GarageLab that simplifies using it, because with just one command, you can define the direction and the speed with up to 2 DC motors.