I've done the old, desperate WiFi hunt, and it is not cool. I appreciate and try to seek out any tips on finding free WiFi, so if an iPhone app can do it — for free, no less — then I am downloading that sucker immediately.
WiFi Finder is a free iPhone App that finds the nearest available WiFi, but it's not just limited to finding free connections, because sometimes you're in such need that you don't care if you have to pay.
But if you do only want to see the free WiFi, just tap the "Free Only" button and you'll see all your gratis options. Be it apps or WiFi, free is generally always a good thing.