This may only be of interest to civil engineers and planners, but topographic maps generated by the U.S. Geological Service are public domain because tax payer monies are used to generate the maps. However, map services have charged for these maps because it costs money to print and distribute the physical copies.
This has changed somewhat recently when a collection was taken up by these guys to liberate these maps. The maps are now distributed by the Internet Archive and can be downloaded for free from this “temporary” directory. It is organized by state, and the maps are in TIF format. XML meta-data files area also available.