How can I search for and order historical USGS topographic maps?

In 2010-12 the USGS scanned its entire library of topographic quadrangle maps.  Nearly 180,000 maps have been published in GeoPDF and are available to the public free of charge. This set of GeoPDF files is referred to as the Historical Topographic Map Collection (HTMC).

Learn more: The National Map: Historical Topographic Map Collection

HTMC maps can be downloaded through three different interfaces:

Downloading the GeoPDF data file is free, and it's easy to view it on a PC or Mac using free software.

Ordering a printed version will cost the price of the map ($15 each) plus handling and any shipping charges.