inSSIDer - inSSIDer Essential makes it easy to maintain small business WiFi.
NetSpot - NetSpot is a free app for wireless site surveys, Wi-Fi analysis, and troubleshooting
Kismet - Kismet is an 802.11 layer2 wireless network detector, sniffer, and intrusion detection system.
Homedale - With Homedale you can search for Wi-Fi / WLAN Access Points and monitor their signal strength.
WiFi Explorer - WiFi Explorer is a tool to scan, find, and troubleshoot wireless networks.
TamoGraph Site Survey - Wi-Fi site coverage survey and modelling application