The David Rumsey historical maps “Featured Content” layer seems to be very popular. What some people don’t realize is that it is possible to overlay any historical map image you have yourself. Soren Johannessen of Copenhagen, Denmark certainly has figured it out. He has posted a collection of seven maps of Denmark (2.4 Mbytes) from 1570 to 1873 showing the progression of map knowledge. This clearly illustrates the improvements in cartography accuracy over time. Just turn on each map one by one to see the different maps. Turn on the “Borders” layer to see how accurate/inaccurate the maps are compared to the modern day maps. Excellent work Soren!
For tips on creating your own image overlays, read this entry from the Using GE Blog. And, here are basic instructions on creating image overlays from the GE 4 user guide.