What a cool idea - Making invisible wireless networks visible by chalk markings. The guys at Warchalking explain it like this:

"What is all this about? Well - it was born out of a good lunch, like lots of ideas; where we hit upon the idea of creating a hobo language for wireless access points.

Find a node, and leave a chalk symbol for others to find the node with a minimum of all that tiresome netstumbler business."

And they have a good designer too. Check out their reference card.