Yellow Arrows are yet another tagging scheme for hyperlinking real world objects and locations. The idea is that the Yellow Arrow company sells you some arrows, each with a unique code. You stick an arrow on a location and then send a message citing the code on that arrow to the Yellow Arrow web site. The message might be a bit of poetry, some travel advice instructions in a game.
Anybody who sees an arrow can send an sms message to Yellow Arrow, quoting the arrows number, and get back your message. Lonely Planet, who publish the travel guides, are so taken by this idea that they are including six arrow stickers with every copy of the “Lonely Planet Guide to Experimental Travel”.
The CSM has an article about Yellow Arrow and other tagging schemes. So does the Washington Post.