In dense parts of San Francisco where this would be most useful, by the time I'm getting into my car, usually someone is already waiting for the space.
This summer, San Francisco is launching a pilot program with wireless sensors that will also produce a data feed that can be used to find parking:
Rutgers Winlab is has also been testing a sensor based approach to detecting open parking spots. They're using a sensor on the car to detect open spaces and measure their sizes to see if they're suitable as parking spots. The professor leading the project
There is an article that talks about it here:
http://www.technologyreview.com/communications/24497/page1/
and the lead author of a paper explaining it is here, in case anyone is interested in reading about it. The paper title is ParkNet.
http://www.winlab.rutgers.edu/~suhas/
This summer, San Francisco is launching a pilot program with wireless sensors that will also produce a data feed that can be used to find parking:
http://sfpark.org/how-it-works/data-feed/