In 2005, two Dutchmen named Jeroen Koolhaas and Dre Urhahn came to the favelas (slums) of Rio to make an MTV film about hip hop. After the filming was done, the two decided to stay.
They formed an organization called Favela Painting to bring "outrageous works of art to unexpected places." Their goal is to paint an entire hillside favela in the center of Rio, visible to all inhabitants and visitors.
Currently they are transforming the favela of Santa Marta in the heart of Rio, with the help of local youths.
More about their work here. You can follow Favela Painting on Facebook here.