Warchalking
Source: Wikipedia: The Free Encyclopedia
http://en.wikipedia.org/wiki/Warchalking: “Warchalking is the drawing of symbols in public places to advertise an open Wi-Fi wireless network.
“Inspired by hobo symbols, the warchalking marks were conceived by a group of friends in June 2002 and publicised by Matt Jones who designed the set of icons and produced a downloadable document containing them. Within days of Jones publishing a blog entry about warchalking, articles appeared in dozens of publications and stories appeared on several major television news programs around the world.”