DavidRumsey.com contains over 59,000 maps and images including rare 16th-20th century maps of North & South America. A new app called Georeferencer allows you to overlay historical maps onto modern maps, helping to reveal changes in boundaries and other features. Rumsey maps has a collection of historical maps available for Google Earth as layers that can overlay topography or map views. LUNA, a special browser interface on the website, has many special features to allow you to work with the maps in ways that make it easier to create with them and share them.
David Rumsey Map Collection