inSSIDer is an award-winning free Wi-Fi network scanner for Windows Vista and Windows XP. Because NetStumbler doesn't work well with Vista and 64-bit XP, we built an open-source Wi-Fi network scanner designed for the current generation of Windows operating systems.
http://metageek.net/inssider
How can inSSIDer help me?
* Inspect your WLAN and surrounding networks to troubleshoot competing access points.
* Track the strength of received signal in dBm over time.
* Filter access points in an easy to use format.
* Highlight access points for areas with high Wi-Fi concentration.