MacOSXHints publishes a tip about using iPhone's Google Maps to find Wifi spots in your area.
So, let's say that I'm in a neighborhood that I'm not familiar with. I ask somebody nearby what the local zip code is (let's say, 92116), and then I go to the Maps widget on my iPhone. I type in wifi 92116 and then hit Search. Up will pop up a whole array of (usually) free wireless networks -- coffee shops, libraries, book stores.
We previously posted a link to Jiwire which provides a similar service, and allows filtering by "Free" Wifi hotpots. Google's method appears to show both free and paid hotspots.