The New York Public Library has released about 20,000 high resolution scans of maps in their collection under a "CC0 No Rights Reserved" license. In other words, they dedicate the scans to the public domain to the full extent they're able to by law... Do whatever you want with them!

The maps include

Through these projects, we’ve built up a great collection of: 1,100 maps of the Mid-Atlantic United States and cities from the 16th to 19th centuries, mostly drawn from the Lawrence H. Slaughter Collection; a detailed collection of more than 700 topographic maps of the Austro-Hungarian empire created between 1877 and 1914; a collection of 2,800 maps from state, county and city atlases (mostly New York and New Jersey); a huge collection of more than 10,300 maps from property, zoning, topographic, but mostly fire insurance atlases of New York City dating from 1852 to 1922; and an incredibly diverse collection of more than 1,000 maps of New York City, its boroughs and neighborhoods, dating from 1660 to 1922, which detail transportation, vice, real estate development, urban renewal, industrial development and pollution, political geography among many, many other things.
The maps are available for online browsing and download.

Open Access Maps at NYPL | The New York Public Library