If your car didn't come with a backup camera, or one with a proximity alarm so you don't hit the guy behind you while reverse parking, why not make your own? This project uses a Raspberry Pi as the brain of a backup camera with a distance sensor that will display the word "STOP" on an attached LCD when you're within 30cm of an object.
You can see a brief demo of the system in the video above. You'll definitely some electronics familiarity to make this one yourself, but we can help you get started there.
The best part of the project is that it ony requires common parts. Jeremy Blythe, the man behind the build, used a Raspberry Pi, a 2.2" LCD from Adafruit, a Sharp distance sensor, and a Microsoft Lifecam camera (which connects to a USB port on the board) to put the whole thing together. He documents the build and includes the code you'll need to get the whole thing up and running at the link below.