A L293D is a great motor driver IC used in a wide array of control boards ranging from the Bricktronics Megashield to the Adafruit motor shield.

Ricardo Marquez wrote a great tutorial describing the L293D and showing how to wire up its various pins to control up to six DC motors or 3 servos.

He even supplies the schematics and Eagle files for a DC motor control board that uses the L293D to do its thing. [via Electronics Lab]