Call me biased since I run tours for them, but I'm pretty particular about who I work for and if they chose me as a guide, then you know they have good judgement! Check out Free Tours by Foot, the only free, tip-based walking tours of New York City.
Tours include Greenwich Village, Lower Manhattan, Midtown Manhattan, Central Park, SoHo, Little Italy and Chinatown, Brooklyn Heights and food tours of Lower East Side, Greenwich Village and Chinatown.