WikiMapia is a "wiki meets Google Maps" mashup intended to be used as a digital geographic encyclopedia reference tool. In its current incarnation, WikiMapia is a little rough around the edges, but keep this site on your list of potential teaching tools.
Here's how WikiMapia works: Key landmarks, such as Rainbow Arch in Utah, the Leaning Tower of Pisa in Italy, or the Pyramid of the Moon in Mexico, are identified on the map. Each landmark has a Flickr type notation (this is the wiki part) which anyone can edit or contribute information related to that landmark.
Placeopedia is an open source mashup of Google Maps and Wikipedia. Using this site, students can connect existing Wikipedia articles with their corresponding location on the map, and then make use of the community generated database to "browse, use, or syndicate the whole lot."