Frank points out that the Rumsey Historical Maps are available now as new feature content in Google Earth.
ArcGIS Explorer has had this layer “coming soon” for quite some time (though it was active at the 2006 User Conference).
Shame the development of ArcGIS 9.2 has taken so long as this layer seems to have given Jonathan Crowe an early Christmas present. The uniqueness of ArcGIS Explorer seems to get smaller and smaller every passing week.
Spatially Adjusted 