inSSIDer - inSSIDer Essential makes it easy to maintain small business WiFi.
NetStumbler - Official source of information about Netstumbler.
NetSpot - NetSpot is a free app for wireless site surveys, Wi-Fi analysis, and troubleshooting
Wireshark - Wireshark is a network protocol analyzer for Unix and Windows. It lets you capture and interactively browse the traffic running on a computer network.
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.