American Panorama is an “Atlas of U.S. History” from Richmond University. Right now, it has several interactive historical maps tracing historical trends over time, including ones for “forced migration of enslaved people,” “The Overland Trails,” “Canals,” and “Foreign Born Population.” They have plans to add several more.
I think the one of “Foreign-Born Population” is particularly useful, and I’m adding a direct link to it to The Best Sites For Learning About Immigration In The United States.
I’m adding the entire site to The Best Websites For Teaching & Learning About U.S. History.
Thanks to Google Maps Mania for the tip.