WiFi Scanner - A 802.11 wireless scanner and connection manager for Mac OS X.
NetSpot - NetSpot is a free app for wireless site surveys, Wi-Fi analysis, and troubleshooting
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...
Vistumbler - Vistumbler is a wireless network scanner for Windows Vista and Windows 7. Features:
Wifi Analyzer - Wifi Analyzer is a handy tool which helps you to select a better channel for your wireless router.
WiFi Explorer - WiFi Explorer is a tool to scan, find, and troubleshoot wireless networks.