Yahoo just rolled out a very cool new beta product called MapMixer, which lets you upload your own maps and overlay them on Yahoo's interactive world maps—even if your map doesn't have just the right proportions or perspective.
There are already some great examples online, including the historical lower Manhattan overlay at right. Make sure you zoom out and play with the overlay opacity. Oh, what a little landfill can do...
If you want to play around with MapMixer but don't have your own personal stash of maps, I suggest you click on over to the Library of Congress, which has some great historical maps its American Memory collection.
MapMixer (beta) (Yahoo!)
