Not really sure how to classify Yellowarrow, although their home page provides this handy definition:

YELLOWARROW [NOUN] A COLLECTIVE SYMBOL FOR PERSONAL COMMUNICATION. [VERB] TO LEAVE AND DISCOVER MESSAGES POINTING OUT WHAT COUNTS.

The about section helps explain it some more. Basically, people take the yellow arrow stickers and apply them somewhere that means something to them. Then they text message the unique ID from the arrow sticker & a short message to the project’s number. Other people finding the arrow can send the ID, prefixed with a ‘?’, to the same number to receive back the short message associated with the arrow they are looking at.

I looked in the gallery for arrows that had been placed in bay area cities, but found only San Francisco and Berkeley represented, and with only 13 arrows total. London didn’t even have any (though there are other non-US cities represented). Check it out, and if you’re so inclined why not join and get some arrows of your own to apply wherever you like.