Legal graffiti
Can real graffiti be legal? not really. The outlaw element of hard core graffiti is at the heart of the culture and art. But legal outdoor graffiti exhibits are still amazing in their own right and provide a fantastic canvas for some beautiful work. The 5 Points warehouse block in Queens, NY is a great example...

More photos and info below...

Tons of great photos of 5 Pointz on Flickr
Taking Art to the Streets Outside of P.S.1.
5 Pointz "Institute Of Higher Burnin"
A bit about the history
NY Graffiti & Steert Art

Photos via OtherThings and Darny