If you are thinking of buying a car, then this app will allow you cross check the cars registration number with records held by the US government, thus you can check if the car has been cloned, which could indicate a stolen car.
You can either type in the number plate, or you can just take a picture of it with your camera to save time.
*Update*
This app will also operate in many other countries outside the USA, including India, Australia, South Africa and New Zealand