Here's a really cool quadrature encoder and motor driver for the GM17 Gear motor designed by the micro-specialists at Didel.
The 1066_1 PhidgetAdvancedServo 1-Motor allows you to control the position, velocity, and acceleration of one RC servo motor.
The PhidgetStepper Unipolar allows you to control the position, velocity, and acceleration of up to 4 unipolar stepper motors.
The Sabertooth 2X12 is one of the most versatile, efficient and easy to use dual motor drivers on the market.
SparkFun's collaborative effort with Brian Schmalz makes the EasyDriver stepper motor driver compatible with anything that can output a digital 0 to 5V pulse.