Miniature RC cars, like Zip Zaps, use a simple two channel radio to control the h-bridge in the car. I experimented with interpreting the radio commands by looking at the motor currents. I hit upon using two opto-isolators for each motor to determine which way the current was flowing through each motor. The signal from the opto-isolators is sent to a pair of Toshiba motor controllers (h-bridges) to control gear head motors in servo cases. The result is a larger and more powerful RC car. I use this method to let cheap two channel RCs communicate with microcontrollers.
The RC car in this project is a two channel Road Gear Pro toy from Shopko. It's cost is less than $10 for a radio that can communicte for more than 15 feet.