I heard about this project a while ago then it seemed to die down and today I saw a link to it again. The idea was to collect up all the old photos of SF and put them on a map. There were some people who went around photographing all the buildings. I think New York has something like this too.

Anyway - Use the slider at the top to narrow the date range and the usual google maps interface to zoom in.

http://www.oldsf.org/#ll:37.781604|-...|-122.41753|11