This is a video introduction to www.historypin.com which might just prove to be a very useful and important project.  It's historical geography powered by collaborative mapping that is infused with social media dynamics.  Backed by Google, they are geo-tagging old photos to recreate the historical geographies of all places and comparing them with current street view images.  You can search by topic, place or date...this has the potential to be very big.   


Via Seth Dixon