iStumbler allows you to find and determine the strength of WiFi networks. It works only with Macs.
iStumbler is useful when you need access to the internet and need free WiFi. iStumbler will tell you if the network is secure or open. Although connecting to an open WiFi point is of questionable legality, there are some networks from coffee shops and other locations that welcome you.
iStumbler is also useful in setting up wireless networks to see where best to place your access point.
While you can just use the built-in AirPort connection information, iStumbler provides you the signal strength information that might help you.