Similar things have already been tried with the iPhone, and all-but failed. Now, Google is attempting to log available parking spaces with Open Spot. This applications requires significant user adoption for it to become a real hit, but if it does, it could prove awesome.
The general principle is as follows. Should you find a parking space in NYC (for example) and leave, you can tag the spot as available on Open Spot. This means that other Android users who are also using the software can tell it has become available and give it ago themselves. Then, once they are done, they tag it as open once more and the process is repeated.
Of course, there are some huge downsides to this. First, it requires a mass-adoption before many parking spaces will become tagged. Personally, I can think of all sorts of different ways to incentivsed users about being prompt with their tagging of available spots. Perhaps the more spots they tag as now available, the more spots they have access to. Anyway, second, and the real clincher, will be whether or not some random pleb takes the spot before an Open Spot user can reach the target destination. Then, they find that they cannot park there anyway, having just battle their way through traffic to get there.
Tough one to pull off, but it is a nice idea.