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.
Packet Sender - Packet Sender can send and receive UDP, TCP, and SSL on the ports of your choosing.
SmartSniff - SmartSniff is a packet sniffer that capture TCP/IP packets and display them as sequence of conversations between clients and servers.
Portfwd - Portfwd, Port Forwarding Daemon, stands for port forwarding daemon.
netcat - Netcat is a featured networking utility which reads and writes data across network connections...
socat - socat is a relay for bidirectional data transfer between two independent data channels.