The Pop Down Project aims to empower guerrilla activists with the power to fight what they call public space pollution.
Participants in the Pop Down Project just download the red-and-white stickers and affix them to the top corner of outdoor advertisements, billboards and other public eyesores. In doing so, the public space pollutants look just like a pop-up box on your browser.
As URLesque points out, it's far easier to ignore a pop-up ad on your computer than a giant billboard on the street. The Pop Down Project aims to make these public ads similarly irrelevant.