NetSpot - NetSpot is a free app for wireless site surveys, Wi-Fi analysis, and troubleshooting
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.
inSSIDer - inSSIDer Essential makes it easy to maintain small business WiFi.
AirGrab WiFi Radar - AirGrab WiFi Radar is a WiFi scanning tool that is used for showing information about Apple AirPort based stations and other WiFi connections and access points.
Kismet - Kismet is an 802.11 layer2 wireless network detector, sniffer, and intrusion detection system.
Lizard Systems WiFi Scanner - Lizard Systems WiFi Scanner is a multi purpose WiFi scanner and WiFi troubleshooting tool designed for dealing with all WiFi related issues conveniently.