You can overlay any historic map in Google Maps or Google Earth, but getting everything to match up accurately through geo-referencing can be quite tedious. In some cases others have already done the hard part, making available free downloads of historical maps sized, geo-referenced and ready for you to import directly into Google Maps or Google Earth.
David Rumsey Map Collection for Google Maps
Over 120 historical maps from the David Rumsey collection of more than 150,000 historical maps have been georeferenced and made freely available in Google Maps, and as a historical maps layer for Google Earth.
Historic Map Works: Historic Earth Overlay Viewer
Historic Map Works features over 1 million maps from around the world in its collections, with a focus on maps from North America. Several hundred thousand of the maps have been geo-referenced and can be viewed for free as historical map overlays in Google through their free Historic Earth Basic Overlay Viewer. Additional features are available through a Premium Viewer available to subscribers only.
Scotland Historical Map Overlays
Locate, view and download free Ordnance Survey maps, large-scale town plans, county atlases, military maps and other historical maps from the National Library of Scotland, geo-referenced and overlaid on Google maps, satellite and terrain layers. Maps date between 1560 and 1964 and relate primarily to Scotland. They also have maps of a few areas beyond Scotland, including England and Great Britain, Ireland, Belgium and Jamaica.
New York Public Library Map Warper
The New York Public Library has been working to digitize their huge collection of historical maps and atlases for more than 15 years, including detailed maps of NYC and its boroughs and neighborhoods, state and county atlases from New York and New Jersey, topographic maps of the Austro-Hungarian empire, and thousands of maps of US states and cities (mostly east coast) from the 16th to 19th centuries. Many of these maps have been georectified through the efforts of library staff and volunteers. Best of all, those which haven't are available for you to georeference yourself through their cool online "map warper" tool!
Greater Philadelphia GeoHistory Network
Visit the Interactive Maps Viewer to view selected historic maps of Philadelphia and surrounding areas from 1808 through the 20th century—plus aerial photographs—overlayed with current data from Google Maps. The "crown jewel" is a full-city mosaic of the 1942 Philadelphia Land Use Maps.
British Library - Georeferenced Maps
More than 8,000 georeferenced maps from around the world are available online from the British Library—just select a location and map of interest to visualize in Google Earth. In addition, they offer a great online tool that allows visitors to georeference any of the 50,000 digitized maps they have online as part of this project.
North Carolina Historic Map Overlays
Selected maps from the North Carolina Maps Project have been geo-referenced for accurate placement on a modern-day map, and made available for free download and viewing as Historic Overlay Maps, layered directly on top of current road maps or satellite images in Google Maps.
Atlas of Historic New Mexico Maps
View twenty historic maps of New Mexico, annotated with descriptions by the mapmakers and by other people living, working, and exploring in New Mexico at that time. Click on the thumbnail of each historic map to view it in Google Maps.
RetroMap - Historical Maps of Russia
Compare modern and old maps of Moscow and Moscow region with maps from various regions and eras, from 1200 to present-day.
HyperCities
Using Google Maps and Google Earth, HyperCities essentially allows users to go back in time to create and explore the historical layers of city spaces in an interactive, hypermedia environment. Content is available for a wide number of locations around the world—including Houston, Los Angeles, New York, Chicago, Rome, Lima, Ollantaytambo, Berlin, Tel Aviv, Tehran, Saigon, Toyko, Shanghai, and Seoul—with more to come.