netcat - Netcat is a featured networking utility which reads and writes data across network connections...
Scapy - Scapy is a powerful interactive packet manipulation program.
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.
Nipper - Nipper - Neat Internet Protocol Packet EditoR - is a graphical packet generation tool.
SmartSniff - SmartSniff is a packet sniffer that capture TCP/IP packets and display them as sequence of conversations between clients and servers.
Packet Editor - Packet Editor a free packet editing tool to debug winsock based network solutions.