iperf - A TCP, UDP, and SCTP network bandwidth measurement tool
netcat - Netcat is a featured networking utility which reads and writes data across network connections...
JPerf - This project gives a better UI and new functionalities to the initial JPerf 1.
PortMapper - Do you need to map a specific TCP or UDP port on your machine to another port on another machine?
NetStress - NetStress is a user-friendly benchmarking tool that is mostly used by networking professionals to measure the network performance of both wired and wireless connections.
Komodia Relay - Free open source tool to perform TCP based session relay