Ooh I like this: The National Library of Scotland has released Spy Viewer, a tool showcasing a set of historic which maps have been georeferenced so that they line up perfectly on top of the current maps. Using a circular spyglass interface you can watch the old maps peek through.
Pictured below is a historic glance at London’s Tower Bridge:
Don’t let the fact that it’s a tool by the National Library of Scotland fool you: other maps of other parts of the world are also available (select yours using the category dropdown)