Neighborhood Boundaries by Tom Taylor uses Flickr Shapefiles and Yahoo! Geoplanet "to show you where the world thinks its neighbors are." Yahoo! provides access to the Where on Earth (WOE) database, which attempts to describe locations as a hierarchy. For example - a town belongs to a city, a city to a county, a county to a state. The Flickr API stores shape files identified by the WOE ID. Here's the punchline. The shapefiles are built using only the latitude and longitude from geotagged photos on Flickr. There's no GIS involved here.
Why this matters, I can't really say. I think it's mostly to show how much data is stored in geotagged Flickr photos. I'm no GIS expert though. Anyone care to comment on the significance?