NetStumbler
Netstumbler is absolutely brilliant for Windows wireless users. It is a full featured tool that can find wifi SSID’s. This knowledge alone will allow you to connect to an open node, if you know its there.
Sometimes people will drive around with a device thats running NetStumbler to map a section of the city and see what signals are avaialbel out there. This is known as wardriving.
this could also be used to verify network configurations. You can test your own and see what other networks are using.
You can find other causes of wireless interference. If you can pinpoint it you can change channels or move your access point.
You can track down and hopefully stop “rogue” access points and scoundrels bridging your connection.
If you are running WindowsCE get ministumbler. If you are a Mac user, iStumbler is for you.