TamoGraph Site Survey - Wi-Fi site coverage survey and modelling application
NetSpot - NetSpot is a free app for wireless site surveys, Wi-Fi analysis, and troubleshooting
Waircut - Free network audit app
Vistumbler - Vistumbler is a wireless network scanner for Windows Vista and Windows 7. Features:
Homedale - With Homedale you can search for Wi-Fi / WLAN Access Points and monitor their signal strength.
Kismet - Kismet is an 802.11 layer2 wireless network detector, sniffer, and intrusion detection system.