Solarbotics Compact L298 Motor Driver allows control of 6 to 24 volt motors, with up to 4 amps of drive current. It comes in kit form. I assembled the unit and have attached photos of the test set-up and a video so you can see it in operation (the photo of the driver by itself is from the Solarbotic's site). Four LEDs indicate motor direction. The driver uses eight Shottkey diodes to shunt back EMF generated by the motors.
With a Basic Stamp, you can control motors of higher voltage with 5 volt signals and simple programming. The code I used was a test sample available on the Solarbotic's web site. The code runs both motors forward, runs both in reverse, stops each motor separately and then runs one forward and the other in reverse and vice versa. It is both simple, yet amazing.