WikiMapia is a Google Maps mash-up that combines a wiki and with a Google Maps interface. WikiMapia allows you to mark physical locations on a map and add descriptive content about the location. The interface is designed as a wiki, so anyone can add or edit information. It also has tagging features, so you can also add tags to your placemarks, and search for locations by tags.
Being a Google Map interface, all you need to do is navigate to a specific location, click the add new link at the top of the map, and then manipulate a resizable box to the exact location you wish to describe. (the box has a similar interface to the "notes" feature in Flickr...) Click the show information link on your placemark box and you are presented with a form where you add your information. The example below is the neighborhood around Lewis Elementary. I added the content about Lewis, but the information about Reed College was added by someone else.
A great tool for describing places in the world or your neighborhood. I wonder if the developers are going to make the code available. It would be nice to run this in a more protected state with students. Would be great for describing and documenting our neighborhood.
By way of Google Maps Mania