If you have a WLAN and don't want to let everyone use it, be sure to secure it (see WirelessNetworkSecurityHowto and WirelessNetworkSecurityNotes). 802.11b is inherently insecure. WEP, invented in attempt to add a little privacy to WirelessNetworks, is also flawed and in fact trivial to break if you simply listen to enough data. Add to these the fact that many of those who run a WLAN are more or less incompetent, and it's fairly easy to find open networks using simple tools, which allows you to surf from the curb. All you need is a laptop with a wireless NIC and some software such as
A GPS receiver is very useful to keep track of where the AccessPoints are. With it you can easily generate maps of open WLANs denoting their position, name, signal strength, WEP status etc. An example can be seen at JWMP.