This is rather spiffy. Google has “officially” added some antique map layers to Google Earth so you can compare todays sat photos with maps of places from a hundred or so years ago.
To use this, expand the Featured Content -> Rumsey Historical Maps in the Layers panel.
While this is an official Google service, they are not the first people to do it.
The rather good blog, Digitally Distributed Environments did a similar thing some time ago (and they used an older map!).