If you are interested about parking radars, or any short range distance or object sensing, this is the right project for you. The author uses the SRF04 sonic module for ultrasound generation and reception, the controller used is ATmega128, but probably a smaller one would also do the job. The device is nothing less nothing more then a radar system, measures the time between the sent sound wave and the reflection, then calculates the distance.
Is is easy to extend this project to a parking radar for your car, not to mention the great potential it has in robotics. The schematics, source code is available for download.
Sonic distance measurement: [Link]