Warchalking 

The practice of marking a series of symbols on sidewalks and walls to indicate nearby wireless access.
Typically with the SSID written above the symbol and Bandwidth below.
Inspired by the practice of hobos during the Great Depression to use chalk marks to indicate which homes were friendly.

I warchalked the secret government agency down the street!

Warchalking by Diamonion February 13, 2004

warchalking 

a practice in which hackers draw a symbol in chalk on a wall or pavement to indicate the presence of a wireless networking node.

warchalking by none September 9, 2002

war chalking 

The practice of leaving coded chalk marks on or near buildings or sites where a "war driver" has identified available 802.11 wireless network access. The marks usually indicate the type of access and level of security.

This practice is decended from the way hobos used to chalk mark the curbs outside homes where an easy mark could be approached for food or other handouts.

I located a particularly handy open wifi network near the ballpark, so I warchalked it for my hacker brothers and sisters.

war chalking by SYFer August 9, 2003