inSSIDer is a free Wi-Fi network scanner for Windows Vista and Windows XP.
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
Export Wi-Fi and GPS data to a KML file to view in Google Earth
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[Via gizmo's freeware]
AirSnare is another tool to add to your Wireless Intrusion Detection Toolbox. AirSnare will alert you to unfriendly MAC addresses on your network and will also alert you to DHCP requests taking place. If AirSnare detects an unfriendly MAC address you have the option of tracking the MAC address's access to IP addresses and ports or by launching Ethereal upon a detection.
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[Via Avinashtech]