The Lionel Pincus and Princess Firyal Map Division of the New York Public Library has released more than 20,000 maps as high-resolution downloads for free. Specifically, the library is distributing all of these images — according to them, the maps have no known United States copyright restrictions — under a Creative Commons CC0 1.0 Universal Public Domain Dedication. The maps can be viewed at the library’s Digital Collections page, and downloaded using the NYPL Map Warper tool. The available collection is mostly assembled from various maps of New York City, but includes many others.
images via New York Public Library