Wikimapia: a Google Maps / Wiki mashup
The short of it: a Google Maps implementation that allows users to add arbitrary regions to areas of the map. You can add a brief title, a free text description, and folksonomy tags to each region. It's multi-lingual too.
At first I was dubious, but then I focused in on
my 'hood and saw how much was not yet described. The result is highly addictive.
I like this concept because not only can you fully name a place, including arbitrary places, but add context as well. The implications for passing on local tacit knowledge are exciting. Network effects should help keep the signal-to-noise ratio low, but a reputation component would be a nice way to enhance the overall quality of the submitted information and reduce the tragedy of the commons effect. We'll see where this goes.