You may have already seen the Rumsey Historical Maps Layer that is included under the Gallery Layer with Google Earth. But, you may not know that David Rumsey has also created dozens of other maps for Google Earth that are not included in the default layer in Google Earth. These can be accessed by the KML link below.
These maps typically date before the 1850s and cover many areas of the world. The collection will automatically update as new maps are added.
1000′s of other historical maps are available at the David Rumsey Map Collection website.