Finding a space in a large parking lot is slow, and sensors at every space are expensive. Most lots already have cameras, and every car has a license plate. ITRI also had a license-plate recognition system that needed an application.
I built a scale-model parking lot with an Arduino-driven camera turret. The system used computer vision and the plate recognizer to track parked cars and free spaces, and a Ruby on Rails website helped drivers find their car or an open spot.