Very Slow Electric Motor
DC motors don't work well at low RPMs.  They stall and have horrible torque. (i.e. they can't turn very hard)  So people have created gear motors: motors with integrated gearing.  The result looks like a slightly bulkier motor, but one that has low RPMs and high torque.  If you were to take apart a running a gear motor, you'd see the motor part actually runs at several thousand RPMs, but it's geared down to something like 60 RPM max.  

A common specialized one is the standard hobby servo, which has some additional electronic bits but is fundamentally a gear motor.  Check out any place that sells motors for robotics or surplus electronics and you'll see several different gear motors to choose from.

DC gear motors are controlled just like normal DC motors, so an Arduino motor shield works just fine with them.