NetSpot - NetSpot is a free app for wireless site surveys, Wi-Fi analysis, and troubleshooting
inSSIDer - inSSIDer Essential makes it easy to maintain small business WiFi.
Wifi Analyzer - Wifi Analyzer is a handy tool which helps you to select a better channel for your wireless router.
NetStumbler - Official source of information about Netstumbler.
Kismet - Kismet is an 802.11 layer2 wireless network detector, sniffer, and intrusion detection system.
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...