Warchalking
Warchalking is the drawing of symbols in public places to advertise an open Wi-Fi wireless network.
Inspired by hobo symbols, the warchalking symbols were conceived by a group of friends in June 2002 and publicised by Matt Jones. The word is ultimately a reference to the 1983 film War Games. The film showed crackers using software to dial randomly selected telephone numbers in the hope of finding a modem. This technique became known as wargames dialing (sometimes erroneously identified as wardialing; see demon dialing), from which warchalking was later derived.
Having found a Wi-Fi node, the warchalker draws a special symbol on a nearby piece of public furniture, such as a wall, the pavement, or a lamp post. Anyone initiated in the ways of warchalking will recognise what it means and get online.