inSSIDer - inSSIDer Essential makes it easy to maintain small business WiFi.
Homedale - With Homedale you can search for Wi-Fi / WLAN Access Points and monitor their signal strength.
NetSpot - NetSpot is a free app for wireless site surveys, Wi-Fi analysis, and troubleshooting
Waircut - Free network audit app
TamoGraph Site Survey - Wi-Fi site coverage survey and modelling application
Ekahau HeatMapper - The Free Wi-Fi Coverage Mapping Site Survey Tool