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.
TamoGraph Site Survey - Wi-Fi site coverage survey and modelling application
tcpdump - tcpdump is a common packet analyzer that runs under the command line.
Waircut - Free network audit app
SmartSniff - SmartSniff is a packet sniffer that capture TCP/IP packets and display them as sequence of conversations between clients and servers.
Homedale - With Homedale you can search for Wi-Fi / WLAN Access Points and monitor their signal strength.