one of the artists, who makes urban interventions with QR codes is Sweza. his project Graffyard creates a connection between the real and the virtual world with a QR code, which is placed exactly on the same place, where the actual street art work used to be. then with QRadio you can use your smartphone as a cassette tape. scan the QR code on the ghetto blaster and access the track on Youtube.
Free Art and Technology Lab also did two interesting projects. with QR_Stenciler a free, fully-automated utility you can converts QR codes into vector-based stencil patterns suitable for laser-cutting. and QR_HOBO_CODES project warns people about danger or clue them into good situations in the urban environment. the QR codes give you hints like "bad coffee", "bike thieves", "cops active", "free wifi" and so on.
in the QR code/urban art the interactivity is crucial - one must be curious enough to approach the code and scan it to discover the surprise. you never know, what awaits you… see you on the "other" side!
check also a nice report about Sweza by Arte Creative, a platform, which explores, shares and remixes interesting contemporary audiovisual creations.