For Your Geographers/Cartographers/Historians: Georeferenced Maps
Link 1: Explore georeferenced maps - Map images - National Library of Scotland
This site allows for the overlay of an old map on top of a new one and comparing them directly. The oldest map available on the site is from 1560 and while the primary focus of the map library is Scotland it also covers including maps of England, Ireland, Great Britain and Belgium.
Link 2: About our Map images - National Library of Scotland
Comparing two maps: Zooming in and mixing the view so that both maps are visible