Public art is often sponsor by the community and supports usually local artists. Sometimes, for example in economically weak areas in portugal or Spain, it is even used to support the local economy at the whole . Of course, in large cities the public art it happens that the public art is supposed to be prestigous and therefore internationally famous artists are asked to created something. Unfortunately this contracted art often costs a lot and its gets the artists idea very seldomly across to the common public. Most art in public places are abstract sculptures, religious sculptures or decorative fountains. the biggest decorative fountain I have ever see was in front of a casinon in Las Vegas. Unfortunately playing in online casino, I have no time to go to Vegas again and I have photographed several example for these different kind of art in public places around my hometown.
Happenings, like Flash mobs or disguising buildings, should be considered as art in public spaces as well. Unfortunately I have not seen it yet. If I will I will add the pictures of it.