SparkFun Ardumoto - Motor Driver Shield - DEV-14129 - SparkFun Electronics
Be aware that L298 based drivers drop a minimum of 1.8 volts off your input.... so if you put 12 in you'll only get 10... need 12?- put in 14.
(That's the minimum loss: goes to 3v at 1A and almost 5 at 2A)