The USGS has been busy scanning and georeferencing its historical maps as part of the USGS National Geospatial Program (NGP). The USGS Historical Topographic Map Collection currently makes over 98,000 maps dating as far back as 1884 available to the public. The maps are available in GeoPDF format and can be downloaded for free from the USGS store.
There are two ways to access the historical maps. Maps from the USGS can be searched via text-based queries from the USGS Historical Topographic Maps search page. Searches can be performed based on the state, map title, or map scaled. Search results return information about the map, date, and links to the map’s metadata and geoPDF file. Geographic searches can be accessed through the USGS Store: Map Locator and Downloader.
Not all states are represented but newly scanned and georeferenced maps will be added to the site over the course of the next year. The USGS goal is to make all 200,000+ archived maps available online. Users should check back frequently to the site for progress updates. A PDF showing completed and scheduled maps is available here.