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).
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 1066_1 PhidgetAdvancedServo 1-Motor allows you to control the position, velocity, and acceleration of one RC servo motor.