A while back in Chicago, a friend and I tried working on a Google maps API site that was sort of a crowd-sourced parking guide -- we'd set up a Google map that could be marked up with where the best free parking was, and then we'd turn it loose on the Internet to get filled in with information. Unfortunately, our project never got off the ground, but the New York Times covers two different iPhone applications designed to do the same thing: help you find some of that sweet, sweet city parking. PrimoSpot Parking is designed to help you find spots in New York City or Boston -- you punch in your location and parking spots, garages, or bike racks will pop up on the screen with markers according to when they open up. You can also locate good spots while moving with a "driving mode," and there's a feature that will mark your own spot once you find a place.
SpotSwitch is a little more complicated -- the idea is that when you leave your spot, you put a mark on the map, so that others can fill it in, and then when you're looking for your next spot, others will return the favor. This is a much more cloud-style type of application, and it depends on a lot of users, which the app may not actually have at any given time. PrimoSpot seems a little more concrete (apparently they hired college kids to fill in the database, though it's only in those two cities).
But if you feel like you're constantly looking for parking, either app may end up helping you (and it's good to see that some folks with a little more tenacity than my friend and I are ready to tackle this problem). PrimoSpot Parking [iTunes Link] is $1.99, SpotSwitch [iTunes Link] is free.