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.