Strangemaps offers up a map of privately owned public open spaces (POPOS) that you can hang out in when you’re in San Francisco.
I recall someone telling me that New York had a city ordinance that required new buildings (all buildings?) to provide a certain square-footage of public space. If only they required public restrooms…now that would be useful.

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Several of the new buildings in NYC have large, attractive atria, some with snack bars, so you can get a cup of coffee and sit. They often have art, greenery, and even rest rooms. In the old days, buildings could get variances on height if they provided even a few feet public space at sidewalk level, often low stone walls that people might sit on. But then the buildings would attach these spikey metal things. It would still look like public space, but nobody could sit there.
For more on privately owned public open spaces in San Francisco, see the great work of the ReBar Group, an art/activism collaborative that takes its inspiration from Bourdieu, among others.
In general: http://www.rebargroup.org/projects/
In re: POPOS: http://www.rebargroup.org/projects/commonspace/
Jerold Kayden has a book called “Privately Owned Public Space: The New York City Experience” that provides all that good information for you: http://www.nyc.gov/html/dcp/html/priv/priv.shtml