One of my favorite institutions (if you can call it that) in San Francisco is the free farm stand (ironically sometimes called the free market). It is a volunteer run event every sunday at a park in the Mission at which produce from local community gardens and fruit trees as well as leftover bread from a bakery are distributed for free. Located in a park with a playground and a community garden, this event becomes more than just an exchange of product. As it is based on a gift economy instead of monetary exchange of a traditional farmers market (or many other interactions in urban life), it emphasizes instead the interaction of different people in this diverse community.
In relation to this, a good academic reference for this sort of programming could be the social practice art of CCA faculty member Ted Purves, whose work emphasizes gift exchange.