I love RC ESCs. They are electronic speed controllers that you connect to a servo output on a RC receiver or a micro controller, and they give you full step free control over a big motor in both direction. Most of the ones made for RC cars work in the voltage range 6-8.4Volts, but there are several alternatives for people who plan to run up to 24 volt motors in almost constant stall at a gazzilion amps (read robot war contenders).
But I'm the eternal cheapskate, and wants the cheapest there is.. So far I've found this