The Perry-Castañeda Library Map Collection at the University of Texas at Austin has scanned many copyright-free maps. For instance, there are many contemporary reference maps from the CIA, and many historical topographic maps from the Army Map Service.
These aren't geo-referenced (registered); Google Earth doesn't know where to locate them. Users will need to use techniques for properly placing the image in Google Earth.
Files from the following locations may be large and cause problems if imported as-is into Google Earth. Data can be pared down in spreadsheet or statistical software (e.g., limiting to certain feature types) before importing.
Many of these featues are not already labeled or not clearly visible in the Google Earth database, so it can be a good way of adding more information.
The Geologic Atlas of the United States is a set of 227 folios published by the U.S. Geological Survey between 1894 and 1945. Included are maps ready for use in Google Earth. Included in each folio are a topographic sheet and several geology sheets. These are tupically 1:62,500 sheets (15' of longitude coverage), so there is only scattershot coverage around the country.
The map images are clipped of their borders, so the extraneous white space, legend, etc., don't show up in Google Earth and they can be seamlessly tiled.