From fab, some really good stuff, pointing to a rather notable effort using craigslist data:
The Neighborhood Project is creating a map of city neighborhoods based on the collective opinions of internet users. Addresses and neighborhood data are translated into latitude and longitude values, and then drawn on the map. The address and neighborhood data are collected from housing posts on craigslist, and from people filling out the form below. The coordinates are generated using the free geocoder.us. I especially like the very nice zoom effect on the San Francisco map.
for more check here.