History Pin encourages users to upload images containing date and location information. They're then displayed as pinpoints on the interactive Google Map system. It's easy to browse to any location to see what's been posted. There's also a timeline filter that let's you narrow the search to specific time periods. In addition to the map-based images, History Pin features image collections with a large variety of themes, such as "Christmas," "Vintage Cars and Vehicles" and "My Grandparents are Better Than Yours."
What Was There also solicits photos and locates them on a Google Map. But it goes one step further and superimposes the old image atop the current Street View. This allows you to see the snapshot or postcard, then let it fade away to show the contemporary scene underneath. The two views don't always line up exactly, but the system is pretty cool nonetheless.
I checked both sites for items related to the Sacramento region. Not much there, unfortunately. But that's easily remedied. Folks here can join and post on both systems for free.
PHOTO CREDIT: Cathedral of the Blessed Sacrament superimposed on 1045 11th St., Sacramento.