The United States Geological Survey (USGS) is the primary civilian mapping agency in the United States. Founded in 1879, USGS produced print topographic maps from 1884 to 2006. US Topo, a digital topographic map series based in GIS data, replaced printed topographic maps in 2009. US Topo is the current topographic map database for the United States. It is based on print/legacy maps that have been updated digitally on an every three years schedule. Both US Topo and scans of historical topographic maps are available for free download from a number of different online sources.
What is a Topographic Map?
FAQ from USGS
Maps for America by Morris M. Thompson (1987)
3rd edition (1987) online from USGS — this PDF can be very slow to download
The text changed very little between editions — 1979 edition through USGS website
US Topo
Includes important information about differences between the US Topo (2009- ) and the previous printed topographic maps ( -2005)
USGS Historical Map Explorer
Search using a graphical map interface for all historic topographic maps that covered selected location; free download of scanned maps
TopoView
Search using map interface, by place name or quadrangle name; includes all topographic maps from 1884 to the present with free download in multiple formats
USGS Store Map Locator
Another way to look for USGS topographic maps; download options not as flexible as TopoView