Packet Sender - Packet Sender can send and receive UDP, TCP, and SSL on the ports of your choosing.
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.
socat - socat is a relay for bidirectional data transfer between two independent data channels.
Ettercap - Ettercap is a suite for man in the middle attacks on LAN.
PortMapper - Do you need to map a specific TCP or UDP port on your machine to another port on another machine?
SmartSniff - SmartSniff is a packet sniffer that capture TCP/IP packets and display them as sequence of conversations between clients and servers.