Warchalking

The placing of chalk symbols on walls and pavements at places where people can use wireless net access. Collaboratively creating a hobo-language for free wireless networking. It's a simple solution to a difficult problem, and one that has grabbed the attention of the geek world with a frenzy.

Some security experts have raised questions about warchalking saying that it could encourage hacking. Clever system administrators know these threats, and configure against them.

More here, here and here.