David Rumsey Map Collection: https://www.davidrumsey.com/
Alternatively, you can go to the alphabetical list of maps or browse by category. To access either, use the tab called View Collection in the navigation bar at the top of the home page. Now you can narrow your search by specifics to find exactly what you might want. The resulting search screen will show you what is available in the category in which you searched and offer you more options for narrowing down, if you need to.
The site has numerous ways to view and save maps. It also has ways to overlay old maps onto new and to utilize Google Earth and Google Maps in very creative ways. You will want to bookmark this site (or add to your favorites) so you can keep going back!
Map of US.org: https://www.mapofus.org/
Another real necessity for genealogists is the ability to see United States county boundary lines as they changed over time and, although there are many fancier county progression maps online, this website is also a treasure trove of much more. Everything about this site is simple and easy, but don't be deceived. There is an abundance of detail hidden beneath the plain façade.
The home page has no bells or whistles, just a list of states and some explanation of why maps are helpful. Click on any state of interest, however, and let the fun begin. Scroll down to get the interactive map of changing county borders and click the "play" button to watch the counties form. You can hit the "stop" button at any time to read the text and view the list of dates and links to individual counties. These links take you to the Random Acts of Genealogical Kindness website (where volunteers are waiting to help.
But don't stop there. Keep scrolling down on each state's page for additional resources. Some states have highway maps of each county (at press time, however, both Missouri and Illinois had non-working links on their maps) and some states have links to other websites with historical maps. There are also links to many other state maps and atlases.
The navigation bar at the top of the page will take you to additional maps of the U.S. and to a range of historical atlases. You will have no shortage of links to explore on Map of US.org!
These are just two of our favorite online map websites that can help you locate locate your ancestors. Enjoy!