Please excuse the commercial but I wanted to let you all know about our new motor control shield for Arduino. It is called the MegaMoto and is designed for motors from 7V to 28V and up to 13A continuous current. A peak of >30A for a short time has been demonstrated. The unit is fully protected against thermal and current overloads.
I'm sure many users would like to be able to control something larger than the tiny motors you can drive with the L298 etc.