Historical map overlays

Historical maps from David Rumsey Map Collection are available as a .kmz file that can be opened in Google Earth and will overlay the areas they cover. (link)

Here are some additional links:

• List of overlays from About.com (link) 
• Austro-Hungary, 1914 (link) 
• Maps Mania: Mapping the Austro-Hungarian Empire (link)

Mapire (link), on the other hand, is a project of mapping the Austro-Hungarian empire. It contains overlays on top of Open Street Map.