MetaGeek's inSSIDer is a free application for Wi-Fi network scanning. You can easily inspect your own Wi-Fi connection and all surrounding networks, scanning and filtering them. The program offers a wide variety of features, such as troubleshooting competing access points and clogged Wi-Fi channels or monitoring the strength of the signals you receive over time. InSSIDer allows also to highlight access points for areas that have high Wi-Fi concentration.
The results you get can be sorted in a number of ways: by SSID, Mac Adress, Channel or RSSI. Data can be later exported to a KML file and used in Google Earth!
The program has a user-friendly, plain interface that makes it easy even for beginning users to explore its features.