Startup Idea : Park My Car-Rent My parking

It's really frustrating trying to find a free place to park in the middle of the city. Would be great to have a mobile app that could show you free spots around. Could be based on public webcams, data from parking lots or maybe people can somehow show click in the app they are leaving the spot so someone else can come. Tough to keep it real time though. the Idea is allow this service for public parking and private by allowing users to rent their parking while there are at work or traveling, they will have the option to rent their parking for a limited time.