Vistumbler is free opensource software licensed under GPL is a Wireless Access Point scanner for Windows Vista and Windows 7. Vistumbler is a good Security tool for not only to Security Experts but also to users with little technical knowledge to quickly scan and see if there are any potential APs around your place.
In simple terms, Vistumbler, you can say is a nice simple GUI for the netsh command:
c:\> netsh wlan show networks mode=bssid
It scans for the Wireless networks within your local area and gives you a more detailed information about each of the Wireless Access points.
Signal strength and Quality
Authentication (open, WPA2 etc)
Encryption (WEP, TKIP, CCMP etc)
Access Point Network type
Location latitude and longitudes and GPS positioning on Google Map
Vistumbler allows you to group and display APs based on SSIS, Encryption, Authentication etc. It also does plot graphs on the strength of the APs and locate on the Google Map using GPS system.