--Well I'm using PWM motor controllers like the ones sold by hobby shops, used to run R/C cars via radio control. They're designed to take a small input signal and amplify it to run a big ass motor. They're relatively common and not too expensive. The interesting part comes when you try to make the 'front end'; i.e. some sort of 'joystick' or knob to act as the speed controller. For that I've gone with Basic Stamp, but you might want to try something else. There's probably an off-the-shelf component for this but I haven't found one yet...