Philadelphia is home to over two thousand murals, more than any other city in the United States. Much of this development over the past two decades is thanks to the Mural Arts Program, which arranges for teens convicted on graffiti charges to apprentice with mural artists.
The above image is one of many trompe l'oeil illusion murals in the city, but other murals' subjects range from political causes to nature to faces of the community.
Link to Flickr group
Link to Time's photo essay - via BoingBoing
Link to Wikipedia article