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.
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.
Ettercap - Ettercap is a suite for man in the middle attacks on LAN.
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.