The Book of Wireless, Second Edition

Unprotected Private Access Points

There's one more source of accessible Wi-Fi signals that we haven't discussed until now: the ones that leak from wireless home and office networks that your neighbors haven't bothered to protect. It can be awfully tempting to grab a connection to one of those networks, but it's both illegal and unethical, even if it is amazingly easy. In many downtown office districts, high-tech industrial parks, and residential neighborhoods, a casual visitor can often find and log in to half a dozen or more wireless networks within a few blocks.

If you can detect a network signal, you might be able to connect through that network to the Internet, but this is legally considered "theft of service"; network managers and law enforcement ...