Transparent street signs

Two years ago, artist Cayetano Ferrer took photos of the scene behind several Chicago street signs and then pasted the prints on top of the signs to achieve an amazing transparency effect. (As the Wooster Collective blog points out, Amnesty International's recent ad campaign employs a similar technique. And it's also the idea behind the "Transparent Screen" trick for your computer display.)
Link to Ferrer's "City of Chicago" image gallery, Link to Amnesty International campaign (via Neatorama)