Homedale - With Homedale you can search for Wi-Fi / WLAN Access Points and monitor their signal strength.
inSSIDer - inSSIDer Essential makes it easy to maintain small business WiFi.
Vistumbler - Vistumbler is a wireless network scanner for Windows Vista and Windows 7. Features:
NetSpot - NetSpot is a free app for wireless site surveys, Wi-Fi analysis, and troubleshooting
iBwave Wi-Fi - iBwave Wi-Fi software suite brings smart networking solutions to enterprise, education, hospitality, transportation, retail, office, healthcare, public venues, and residential wireless networks.
Waircut - Free network audit app