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
Winsock Packet Editor - WPE Pro is a packet editor.
Vistumbler - Vistumbler is a wireless network scanner for Windows Vista and Windows 7. Features:
packeth - packeth is GUI and CLI packet generator tool for ethernet.
Kismet - Kismet is an 802.11 layer2 wireless network detector, sniffer, and intrusion detection system.