Here's a really cool quadrature encoder and motor driver for the GM17 Gear motor designed by the micro-specialists at Didel.
A 3A, 8-45V bipolar stepper motor driver - this breakout packs a mean punch for it's size! It has built-in overcurrent detection, undervoltage detection, overtemperature detection, stall detection, a 5-bit ADC, and a switch input, microstepping support and PWM drive voltage limiting.
You got some really big motors you want to control? Take a look at the Sabertooth dual 25A motor driver, able to supply 25A each to two motors, at up to 30V. It's a brute, but it's smart enough to talk in many ways!
The Sabertooth 2x5 motor driver is a compact, flexible way to drive motors up to 5A. It talks in 4 methods, so you're sure to be able to use it for your project.
The HB-25 Motor Controller in a fast and inexpensive way to control a DC motor up to 25 Amps.
Just need to make some motors move with your Arduino? Here's a simple way from SparkFun - the Ardumoto!