You've seen them, as you walk in the city. The posters with the striking faces, the angry text, the provocative pose. Maybe you see a half-dozen all in a row; maybe it's alone on the pillar supporting a freeway overpass. You may not be able to decipher quite what it means, but you know it means something.
Street Memes is a new site which (in the words of site editor Ryan Watkins-Hughes) "tracks the spread of stencil graffiti, sticker art, and political posters." Examples include the nearly-ubiquitous Andre the Giant stencils and posters, "Stop the RNC" posters calling for protest in NYC this August appearing all over the city, the mysterious (or perhaps just confused) "Pray for Pills", and more. The pages include links to similar memes, so you can explore the variations of urban art to your heart's content. The content is entirely visitor-contributed; if you've spotted a meme on the street, take a picture and send it in. The site currently has a distinct New York dominance, but since I've seen similar art up on the urban walls of San Francisco, Seattle, and Los Angeles, I expect the collection to grow quickly.
Didn't the Wobblies call stickers and posters "silent agitators?"
Social Design Notes recently has some interesting observations on political graphics: http://www.backspace.com/notes/2004/06/13/x.html et seq.