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

Via Seth Dixon, Jose Luis Redondo, Adriana Marichal