Free the Billboards was a 2012 project started by Nina Montenegro in Portland, Oregon in an attempt to “decapitalize” the city. Montenegro collected from the public reimagined billboards with poems, images, photographs etc. which might replace the adverstisements currently shown. Free the Billboards then installed Portland-made View Masters in front of billboards around the city, which passersbys could look through to see the reimagined images transposed over the existing ad. 
Source: http://eng.partizaning.org/?p=4382