Do you live in fear of speeding tickets? I no longer drive, but when I did, they were the bane of my existence. Too bad I wasn't a driver in the time of the iPhone, because Trapster is the latest app that I'm declaring brilliant.
With this free app, the theory is that you, as the driver (though I'd make the person riding shotgun do it for safety's sake), will just tap the app's map (which is following you via GPS) to report a cop car hiding out with his radar gun.
Reported cop car/radar gun sightings go to a database, which alerts other drivers with the app in the area that they should watch out. Verrrry sneaky.