inSSIDer is a wireless networking utility that allows you to find wireless networks in your area and control, in a graphics mode, the intensity of their signals. It is a freeware network scanning application that's easy-to-use, well-designed, powerful and advanced.
Here are some screenshots to illustrate the features and user interface of this application:
Homedale WiFi
WifiInfoView
Intel PROSet/Wireless WiFi Software
TekWiFi
Wireless Network Watcher