There are thousands and thousands of great maps on NYHeritage, but I especially like two from Northern New York because they're Sanborn Fire Insurance maps.
The Sanborn Map Company (still in existence today) was one of several companies who sold atlases in the 19th and 20th centuries specifically for the purpose of assessing fire insurance risk. Insurance companies needed to assess details like building material, building density, facility use, nearby fire hydrants, and more in order to figure out rates to charge for fire insurance coverage.
These maps were large scale, at 50 feet to 1 inch (a scale of 1:600), and full of information that frequently changed. To keep up the accuracy of the atlases, the company mailed out corrections with instructions to paste over existing maps. As a result, someone looking at a Sanborn map today can see the growth of a city illustrated by the paste-in corrections, as seen here in the 1926 map of Potsdam:
These maps are interesting to anyone familiar with the city, but they can also be a treasure trove of information for genealogists. Consider that you may know the address of your ancestor, and maybe even the company he or she worked for in Plattsburgh in 1902. You can trace the kind of commute your ancestor would have had! The detail of these atlases brings the past to life: the parks, the streets, the businesses, and, of course, the fire hydrants.