Here is a new guerrilla artist that I've just discovered:
The Polish Artist NeSpoon
Yarn Bombing has made its way around the world. I know I've posted pics somewhere on this blog before, but the Yarn Bombing link is worth a visit.
And Guerrilla Gardening:
below, in Japan (via Cubeme)
Wrap Your Troubles in Dreams)
More about guerrilla gardening:
here, and here, and here -- and dozens of other places...just Google guerrilla gardening!