Billboard Liberation Front
   "Improving" outdoor advertising since 1977. Includes how-to manual for altering outdoor advertising.
   http://www.billboardliberation.com/

Culture Jammer's Encyclopedia: Billboard Liberation
   Includes gallery with dozens of examples of liberated billboards.
   http://www.sniggle.net/Billboard/

Ron English POPaganda!
   Prolific producer of surreal and parodic billboard art since 1982.
   http://www.popaganda.com/

Smashing the Image Factory
   A complete manual of billboard subversion and destruction. Humorously ranges from the innocuous to the terrorist.
   http://www.eco-action.org/dt/smashing.html