This Google Map of Flickr's crowdsourced boundaries is great fun as it allows you to check your notion of area boundaries with Flickr's crowdsourced definition.
Photo sharing website Flickr suggests a selection of place names when users geotag their pictures. These place names are used by Flickr to compute the shape of geographical areas. These area boundaries are then feedback into Flickr to help suggest those place names to users when they geo-tag their photographs.
Tom Taylor has created a Google Map that users can search to view Flickr's area boundaries anywhere in the world. Tom says that "whilst the shapes of places and neighbour results can be inaccurate, in areas of dense Flickr activity such as urban city centres, the results are often excellent."
As Tom points out on his site geographic identifiers are often very fluid. People often have different ideas about the boundary of areas than that held by local and national government, postal agencies and other parties. Using Boundaries you can compare your ideas of an area to those held by 'the cloud'.