L293D Motor Driver Shield
Arduino is a great starting point for electronics, and with a motor shield it can also be a nice tidy platform for robotics and mechatronics. Here is a module designed for a full-featured motor driver shield that will be able to power many simple to medium-complexity projects.
The Motor Driver Shield includes 4 H-Bridge drivers, allowing you to control up to 4 DC motors with independent control of speed and direction from your Arduino UNO, Diecimila, Duemilanove or Freeduino or compatible boards. Alternatively, it will drive two stepper motors, and the PWM capability makes microstepping a breeze! This shield also provides two headers for connecting standard RC hobby servos!