Today I came across metrofreefi.com and found a bunch of cafes and restaurants in Washington, DC, that have free wi-fi. The website lets you search by region and city to find the places near you that offer free wi-fi. This isn't a comprehensive list - DC only had 26 listed - but it asks for submissions. I added the new coffee shop in my neighborhood and sent it in about 2 minutes.
The Wi-Fi-FreeSpot Directory provides a similar service.  About the same amount of spots came up in a search for Washington, DC, but this site listed a lot of hotels and public buildings that I wouldn't likely go to.  WiFi 411 has a different approach.  They list both free and
commercial locations, although pay connections make up the bulk of the hot spots (for DC only 3 free hot spots showed up in a search). What's interesting about the website is that it doesn't just list US locations, like the other two.  More than 70 countries are included in their database, and even though many of these countries only have one or two listed, a lot have more, such as England with 6,538 and Japan with 2,884. And I was blown away to see the Brazil has 255 hot spots listed - that's impressive. All figures as of June 13 at 8:20 pm.