Warchalking is similar to the idea of a hobo language, a secret system of marking out things for others versed in the system to find when they are in an unfamiliar place.
It has overtones of geocaching and maybe bookcrossing but its purpose is to promote open wifi nodes.
From an oreilly page, "You've found yourself an open Wireless Lan - and you want to share the knowledge...do you slap it on the web? Well, that's a good idea, except that to find the node, you have to be on the net in the first place. Not much good if you're just off a plane, in a strange city and need to grab your mail.
So why not WarChalk - Matt Jone's hobo-inspired idea for spreading the wireless word.
You find a node, and draw the correct symbol on a nearby piece of public furniture - a wall, the pavement, the side of a lamppost. Anyone knowing in the ways of the WarChalking will recognise what it means, and get online. No more wandering around bandwidthless, and no more struggling with online maps."