An interesting variation of this street art are the stickers that you could find almost everywhere in Southern Manhattan, on lamp posts, mail boxes, energy transformers and where else.
Stickers are small pieces of paper than be glued placed anywhere accessible. Placing them goes "with a much lower risk of apprehension by officials enforcing anti–vandalism laws", explains Wikipedia (wikipedia). They also report: "Sticker artists can design and print thousands of stickers at low cost using a commercial printing service or at home with a computer printer and self-adhesive labels".
It seems there is a cult around those stickers. There are some websites dedicated to this art form (streetartstickers stickerobot).
Stickers have different purpose. Often they advertise something, like an upcoming rock concert. Others display politics, religion and Weltanschauung. Many stickers are just for poking fun and to provoke.
I like them. They make the city more colorful and cool. Some give me something to think about. Often the combination of different stickers forms a pattern - accidental or not. Those collages are an art form by themselves.
To be continued