This website is the online collection of the David Rumsey Map Center at Stanford University. There are over 91,000 high-quality historical maps on the site for almost every region of the world, dating back to the 16th century. The site also offers browsing by “What”, “Where”, “Who”, and “When”, for if you’re looking for a particular city, person, or time period. Downloads are perfect for georeferencing (and they have their own online Georefrencer v4 application) – most are available in MrSID, SID, JP2, or TIFF formats. Once you’ve selected a map, you can view a preview up-close to see if it contains the information you need before downloading, and you can see its extensive metadata. If you’re looking for historical maps, this is the place to check first!
The National Map’s Historical Topographic Map Collection is a collection of USGS topographic maps printed from 1884 and 2006. They’re in PDF format with GeoPDF extensions, and you can use their download client to find and download the maps you need. If you aren’t familiar with GeoPDF maps, don’t worry, it doesn’t contain any features you won’t be able to intuit from ArcMap. The site offers an optional toolbar called TerraGo for free download which can help for work with GeoPDF (note that like ArcMap, it only works on Windows, but the maps themselves work on all operating systems). These are detailed maps – 1:250,000 scale and larger – and are ideal for comparing changes in geological features over time. Check out the “Fact Sheet” if you don’t have experience with georeferencing USGS quadrangles!
I highly recommend the TopoView application of the USGS Historical Topographic Map Explorer as well, which allows you to zoom in on the geographic location of your choice (by coordinates, name, etc) and scroll through a timeline of the maps the USGS has available for that area. They’re color-coded by the scale of the map, which makes it easy to see which might suit your needs best. The most useful feature, though, can be explored when you click on a map in the timeline. It imposes the map you chose over a modern ESRI map of the area, making it incredibly easy to tell where the borders of the map are in comparison to present day. It will even layer several maps at the same time, and allows you to change the transparency or layer ordering! Check it out – this application is very cool.