From the State Library of Massachusetts:
The State Library has a large collection of bird’s-eye view maps that were digitized and are now available online! These maps illustrate with great detail aerial views of cities and towns in Massachusetts–much like what you can imagine a bird would see flying overhead!–with a few maps from other areas outside of Massachusetts.
The online collection includes 120 maps so far, with many more to be added in the near future. Most maps date from the late 1800s up to the early 1900s.
Hat Tip: Eastman Genealogy Online