These are just a bunch of clever billboards that I found while researching my Independent Project:
Here the natural gradient of the sky has been put to good use with this transparent pantone strip advertising paint.
Here the Army Special Forces have opted for a clever billboard challenging potential applicants to show what they’re made of an reach for a leaflet on the side of the building!
This clever piece of advertising invites people to try and get at the money through the security glass that they offer for sale, to prove its strength - and people have tried!
More clever billboards can be found here: http://www.allgraphicdesign.com/graphicsblog/2008/08/28/best-cool-clever-unique-billboards-and-outdoor-advertising-part-ii/