This is very interesting.

Yellow Arrow is a sticker art/collaboratively authored viral narrative project. Project participants post a yellow sticker pointing to something they think is important in the urban landscape, and then send a short email description to the yellowarrow.org server via their phone. Each sticker has an individual ID. When other users encounter the sticker in the public space, they then can send an email to yellowarrow.org to retrieve the description the tagger left behind.

Be sure to browse the yellowarrow gallery.

[Via Grand Text Auto]