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).
The Sabertooth 2X12 is one of the most versatile, efficient and easy to use dual motor drivers on the market.
Pololu is known for some killer motor driver technology. They're bringing their mojo to the Arduino! Dual robust VNH5019 motor drivers take 5.5 to 24 V and punch out a continuous 12 A (30 A peak) per motor, or a continuous 24 A (60 A peak) to a single motor connected to both channels.
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 1066_1 PhidgetAdvancedServo 1-Motor allows you to control the position, velocity, and acceleration of one RC servo motor.