Found this on ebay - looks interesting for when parking is tight in your garage.
Drive your car into your garage with the confidence you wont hit the back wall.
This unit is your own personal set of traffic lights that tell you when to stop so you are perfectly positioned in the garage.
The Green light illuminates at about 1m- 'Keep driving', the yellow light at about 700mm- 'Get ready to stop', and the red light about 400mm- 'Park it now!!' .
It uses an Ultra-Sonic sensor to measure how far away your car is and is attached with a curly cord. Its easy to install and is supplied with Velcro mounting.