NetStumbler. An ugly but functional free wireless network sniffer program which gives an overview of the wireless networks in your area. You can use it to locate nearby WiFi networks or to optimise your home network by finding out where the signal degrades and the antennas need to be pointed. Nice little tool.
NetStumbler (also known as Network Stumbler) is a tool for Windows that facilitates detection of Wireless LANs using the 802.11b, 802.11a and 802.11g WLAN standards. It runs on Microsoft Windows 98 and above. A trimmed-down version called MiniStumbler is available for Windows CE.