Dagu Mini Driver
Now, wait just a gosh-darn second here... Is this a real-life Arduino board with a motor driver AND a USB connection for 16 bucks?!
Why, yes it is.
Dagu has really hit it out of the park here with everything you would need for a small or medium-sized robot. 2 amps per channel on the motor driver gives you plenty of umpf for your motors, USB on board means no FTDI board to buy. The Atmega8, while it can't store quite as big of a program as a 168 or 328, is still plenty big for just about anything you would want to do with it. Just a great board.
*This is a 5v board
**PWM outputs can not be used simultaneously with the servo library
In the box:
Mini Driver Board (fully assembled, ready to go)