Since starting Hirelite: Speed Dating for Software Jobs, I've been taking a lot of meetings around New York City. After each meeting, I like to find a place nearby to sit, plug in my laptop, and hack for a few hours. In New York, it's hard to find a place you can sit with some elbow room unless you know where to look (Starbucks is usually packed). It turns out, due to zoning regulations, NYC abounds with free public space, often indoors and often with open power outlets where they don't mind you sitting for a few hours.
tl;dr - In Manhattan, look for newish buildings that have a higher base height than buildings around them. Chances are, there's a place you can hack.
[Base height illustrated source]
Example
In the example below, there are three buildings. #2 has a much higher base height in relation to buildings #1 and #3. There's probably a place to hack here.
Why is this?In the late 1800's and early 1900's, as buildings in New York City were built taller and taller, people protested the loss of light due to the shadows cast and light blocked by new buildings. When the Equitable building was built in 1915, it cast a 7-acre shadow and enforced the need for more zoning regulation.
[The Equitable Building source]
Early attempts to regulate building height, base height, and setbacks (how high building walls may be before they must be set back in relation to the building footprint), including the 1916 Zoning Resolutions, were too restrictive and were plagued with other problems inherent to a growing city.
New regulations were drafted to better fit the city's expansion, culminating in the 1961 Zoning Regulations. These regulations set forth incentives for building developers to create publicly available space. Essentially, the new regulations allowed building developers to build with a higher base height in exchange for having an enclosed atrium, an outdoor park, or some other public space. This is where the hack comes from (the hack doesn't work if there are a cluster of buildings taking advantage of these incentives). If you prefer to research for yourself instead of finding a place on the fly, just visit this list of of all privately owned public space in NYC. This will also help you narrow down outdoor vs indoor public space. For more information see: NYC Zoning History.