Here's a really cool quadrature encoder and motor driver for the GM17 Gear motor designed by the micro-specialists at Didel.
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.
The jrk 21v3 motor controller is a highly configurable brushed DC motor controller that supports four interface modes: USB, logic-level serial, analog voltage, and hobby radio control (RC).
Just need to make some motors move with your Arduino? Here's a simple way from SparkFun - the Ardumoto!
Need a tidy, simple, and capable motor controller? The SparkFun breakout board featuring the Toshiba TB6612FNG motor driver can control up to two DC motors at a constant current of 1.2A (3.2A peak).