Of all the different apps that you may have seen for teh iPhone, I bet nothing even comes close to this one. It converts your Apple iPhone into a radio remote control for a RC car. The app even makes fantastic use of the iPhones accelerometer to allow you to virtually drive the car.
Once you launch this RC car controller app on your iPhone, you will be presented with five big buttons across the touchscreen of the iPhone. These five buttons allow you to move your car in forward, backwards, left and right and the fifth one does something special which activates the the accelerometers in the phone when it pressed, which you can then control the RC car by simply tilting the iPhone.
It makes use of a Wifi connection and requires a computer in the loop as well.