NetSpot - NetSpot is a free app for wireless site surveys, Wi-Fi analysis, and troubleshooting
Vistumbler - Vistumbler is a wireless network scanner for Windows Vista and Windows 7. Features:
Wifiner - WiFi Analyzer - WiFi Analyzer. See your WiFi signal on a visually comprehensive map and make sure you are getting the best of your network
Kismet - Kismet is an 802.11 layer2 wireless network detector, sniffer, and intrusion detection system.
WiFi Warden - WiFi analyzer providing details like frequency, channel, modem manufacturer and more.
Wifi Analyzer - Wifi Analyzer is a handy tool which helps you to select a better channel for your wireless router.