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:
WiFi Scanner - A 802.11 wireless scanner and connection manager for Mac OS X.
WifiInfoView - Scans wireless networks in your area and displays extensive information, including: SSID, MAC Address, PHY Type (802.11g or 802.11n), RSSI, Signal Quality, Frequency, Channel Number, and more...
Homedale - With Homedale you can search for Wi-Fi / WLAN Access Points and monitor their signal strength.
Wifi Analyzer - Wifi Analyzer is a handy tool which helps you to select a better channel for your wireless router.