Could I use a powered parachute to fly around New York City? NYC traffic is so horrendous that it would be pretty convenient to be able to take off with a powered parachute (PPC) from the top of a building or from a park and be able to fly above all the traffic to your destination. I'm wondering if anyone has any insight into this.

I know the class B extends to the surface in upper Manhattan but how about downtown? Class B starts at 1100' (plenty of space for a PPC); could one fly around there without talking to anyone? Also, could one potentially get a class B clearance to fly around at low altitudes in upper Manhattan?   

Finally, does anyone know how much space a PPC can take off in? Is there any way a STOL PPC could take off from an NYC rooftop (or more realistically an open space such as Central Park)?

Don't give this question too much thought, just something fun to consider. I think the real limitation would be regs here, even if you could go to the Great Lawn or something to take off.
Pretty simple, powered parachutes fall under the ultralight vehicle category, which is 14 CFR 103:

14 CFR 103.15 Operations over congested areas.:


  No person may operate an ultralight vehicle over any congested area of a city, town, or settlement, or over any open air assembly of persons.


And Also:

14 CFR 103.17 Operations in certain airspace.


  No person may operate an ultralight vehicle within Class A, Class B, Class C, or Class D airspace or within the lateral boundaries of the surface area of Class E airspace designated for an airport unless that person has prior authorization from the ATC facility having jurisdiction over that airspace.


It is highly unlikely that you would get a waiver to fly your PPC in the Manhattan Class-B, even if 103.15 didn't preclude it in the first place. You can fly in Class-B in certain areas, but you do need to be talking to ATC.