Less than two weeks ago, NYC's Department of City Planning removed the paywall from the PLUTO dataset (it used to cost $1,500 for the full set). The PLUTO (Primary Land Use Tax Lot Output) dataset contains land use and geographic data for tax lots across the five boroughs. It's a treasure trove of city data—and now thanks to Andrew Hill and the folks at CartoDB, you can look through all the maps in an orderly fashion here.
If you've ever wanted to know how much land in each borough is city owned, or how tall the skyline of Manhattan really is, or how many buildings were constructed pre-1900, or where people are walking up the most flights of steps (the winner: 16 flights!), you get the drift: you should check out the maps now!