Property Shark was begun in 2003 by a frustrated laid-off programmer turned Harlem landlord as a user-friendly front end to the city's property tax rolls. Since then, its feature-friendly site has become the Google of the real estate set, but it's also a great tool for amateur land-use sleuths, or just the nosy: That guy across the street annoying you? Find out just how much he paid for his house, and how many buildings department violations he's racked up. Or if you're feeling more ambitious, dial up the Maps feature to find all the underground oil spills in your neighborhood, or play Who Gets Gentrified Next? with color-coded charts of where the city's unused air rights are concentrated. (Anyone needing more than six searches a day—in other words, realtors—has to pony up for a subscription; for the rest of us, it's free.) Property Shark has even added photos of every residential building in the city and is working to add commercial properties as well—at this rate, it's only a matter of time before Homeland Security shuts the place down for placing dangerous information in the hands of our enemies.