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.
NetSpot - NetSpot is a free app for wireless site surveys, Wi-Fi analysis, and troubleshooting
tcpdump - tcpdump is a common packet analyzer that runs under the command line.
inSSIDer - inSSIDer Essential makes it easy to maintain small business WiFi.
SmartSniff - SmartSniff is a packet sniffer that capture TCP/IP packets and display them as sequence of conversations between clients and servers.
Kismet - Kismet is an 802.11 layer2 wireless network detector, sniffer, and intrusion detection system.