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.
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).
The PWMPAL provides up to 4 channels of PWM control to any microcontroller.
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).
This is SparkFun's version of the OpenServo open-source servo controller board. It's meant to replace the existing controller board in any standard servo (~.78 inch servo width).