The Neighborhood Project is an "experiment in collective knowledge" that first went live in San Francisco, with plans to roll it out to other cities later. We assume that New York has to be high on the list of cities for future roll-out, given its rich concentration of diverse neighborhoods. From The Neighborhood Project's Website:

"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. The map is from the TIGER/Line US Census data."

Craig Newmark of Craigslist gives it a thumbs up, calling it "a rather notable effort using craigslist data."