Is anyone familiar with this website?
http://www.whatwasthere.com/
It's a site for historical location photos that uses Google Maps as a basis for organization. One can use the map to select any portion of Los Angeles (or whatever city you choose), then select a vintage photo from the list of available images. You can then select "Street View" to see it superimposed on a current image of the same location, and use the "Fade" slider to dissolve between past and present.
I think the site must be fairly new. Right now there are fewer than 100 historical photos available for Los Angeles.
I also discovered an error. There is a 1927 photo of the John C. Fremont Library on Melrose Avenue that is incorrectly matched. They've paired it with a recent photo of a building that is on the opposite side of the street, one block east. The John C. Fremont Library still exists and has changed very little. (I live just one block away from it.)