Scapy - Scapy is a powerful interactive packet manipulation program.
Komodia Relay - Free open source tool to perform TCP based session relay
Winsock Packet Editor - WPE Pro is a packet editor.
socat - socat is a relay for bidirectional data transfer between two independent data channels.
Nipper - Nipper - Neat Internet Protocol Packet EditoR - is a graphical packet generation tool.
PortMapper - Do you need to map a specific TCP or UDP port on your machine to another port on another machine?