Since a few weeks/months the New York Public Library offers over 20 000 historical maps for free. The catalogue mainly covers the US but also includes many historic maps of European cities and entire countries in high resolution which can be downloaded either as TIFFs or as georeferences GeoTIFFs or KLM (e.g. to use it as a layer in GoogleEarth). For example this map displays Germany in 1681.
The maps can be accessed via the NYPL Map Warper: http://maps.nypl.org/warper/