Not only has the New York Public Library put 20,000 hi-res maps online (as well as free to download), they have included a “warping” feature which allows you to overlay historical maps over modern tools.
Not all of the maps have been set up for warping yet so the search capability allows you to filter results for that if you want. I was able to find a map for downtown Rochester in 1838 and view it as an overlay in Google Earth. It was interesting to see how some names of streets have changed over the years while some have remained the same. Accuracy will vary depending on the number of control points they were able to identify and match but it’s still pretty amazing regardless. I’ve been looking for something like this for many years.