inSSIDer is an free, award-winning Wi-Fi network scanner application for Windows Vista and Windows XP.
It scans networks within reach of your computer's Wi-Fi antenna, tracks signal strength over time, and determines their security settings (including whether or not they're password-protected).
NetStumbler, the most popular Wi-Fi network scanner, is free, but it hasn't been actively developed for years, and it doesn't work well with Vista or 64 bit OS.
inSSIDer, on the other hand, works like a charm on both Vista and XP, 32 and 64 bit, and it's open-source.
This must-have for hunting down Wi-Fi networks on the road.
Works with internal Wi-Fi radio
Wi-Fi network information (SSID, MAC, data rate, signal strength, security, etc)
Group by Mac Address, SSID, Channel, RSSI and "Time Last Seen."
Graph 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.
Open source (Apache License, Version 2.0)