Might work as an iPhone or mobile app. Not as an app for standard browsers -- the data are too time-sensitive, and I don't have the coordination to juggle a laptop while driving a car.

What I'd really kill for is a version of this app that could alert me to on-street spaces, or other cheap alternatives. I don't usually have much trouble parking in downtown garages -- they cost so much that they often have spaces, and if they don't have spaces they generally have signs at the entrance to tell you. I once had the bright idea of planting a sort of super-RFID in the pavement under each on-street parking space -- or maybe in the parking meters? -- and monitoring those with an online service. Make that work and I'll be your friend for life.