NetSpot - NetSpot is a free app for wireless site surveys, Wi-Fi analysis, and troubleshooting
WiFi Explorer - WiFi Explorer is a tool to scan, find, and troubleshoot wireless networks.
Wifi Analyzer - Wifi Analyzer is a handy tool which helps you to select a better channel for your wireless router.
Vistumbler - Vistumbler is a wireless network scanner for Windows Vista and Windows 7. Features:
Kismet - Kismet is an 802.11 layer2 wireless network detector, sniffer, and intrusion detection system.
NetStumbler - Official source of information about Netstumbler.