The Propeller Servo Controller allows you to control up to 16 servos (32 using two Propeller Servo Controllers) by sending serial commands from a microcontroller or PC via its USB connection.
The PWMPAL provides up to 4 channels of PWM control to any microcontroller.
The PhidgetStepper Bipolar 1-motor allows you to control the position, velocity, and acceleration of one bipolar stepper motor.
The PhidgetMotorControl LV is a USB based controller that can control two low-voltage brushed DC motors independently for direction, velocity and acceleration.
SparkFun's collaborative effort with Brian Schmalz makes the EasyDriver stepper motor driver compatible with anything that can output a digital 0 to 5V pulse.