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.
Vistumbler - Vistumbler is a wireless network scanner for Windows Vista and Windows 7. Features:
tcpdump - tcpdump is a common packet analyzer that runs under the command line.
Kismet - Kismet is an 802.11 layer2 wireless network detector, sniffer, and intrusion detection system.
Ettercap - Ettercap is a suite for man in the middle attacks on LAN.