inSSIDer - inSSIDer Essential makes it easy to maintain small business WiFi.
NetSpot - NetSpot is a free app for wireless site surveys, Wi-Fi analysis, and troubleshooting
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.
Wifi Analyzer - Wifi Analyzer is a handy tool which helps you to select a better channel for your wireless router.
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.