I’m not sure how I happened across the website MapofUS.org, but it seems like quite a find for history and map lovers, and, of course, family history researchers.
The site consists of links to many, many high-resolution historical maps of the U.S. (and a few other countries), as well as a small selection of battle maps for the Revolutionary War, and Civil War maps promised to come soon.In addition, there is a Historical Atlases section, which provides some city maps as well. I was able to see 1852 and 1880 maps of St. Louis, where I live. They were fascinating.
It also offers an interactive U.S. map where you can watch the territories and states as they’re established throughout time. Each state also has an interactive county map.
The maps are available free of charge (at this point anyway). This seems like a really easy way to check county (and other) boundaries on various dates while you’re doing your genealogy research.