retrolefty:
Pololu has many standalone motor controller boards and is a pretty popular supplier of such, but don't know if they have an in the 'shield' form factor.
Pololu does make a couple of motor driver PCBs that can be directly plugged into an Arduino's headers like other shields. Although both don't have the exactly same form as a standard sheild (i.e. they didn't bother to include the cut-outs at one end so these boards are plain rectangles, and one doesn't completely cover an Uno). Appearance aside like most motor shields they can control up to two motors, and like some are designed around an IC with an H-bridge and additional logic (as opposed to just a high power transistor). The more expensive one can handle a relatively large amount of current, 12 A continously for each motor.