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.
SmartSniff - SmartSniff is a packet sniffer that capture TCP/IP packets and display them as sequence of conversations between clients and servers.
Winsock Packet Editor - WPE Pro is a packet editor.
tcpdump - tcpdump is a common packet analyzer that runs under the command line.
packeth - packeth is GUI and CLI packet generator tool for ethernet.
Packet Sender - Packet Sender can send and receive UDP, TCP, and SSL on the ports of your choosing.