usbcar... Dream Cheeky sells a USB controlled RC Mini Car. It comes with software for Windows, but here is a program that lets it work with Mac OS X.

The software is quite simple - click a button to send the command to the car, click it again to deselect that button and stop the car. You may also drive using the arrow keys (note that left and right do nothing on their own, they only work in conjunction with forward and back). If the status indicates that the usb device is not found, you must plug it in and restart the software (the software does not auto-discover the device when it is already running)

The USB RC Mini Car is here at the Dream Cheeky Site.