iperf - A TCP, UDP, and SCTP network bandwidth measurement tool
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.
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.
TamoSoft Throughput Test - TamoSoft Throughput Test is a utility for testing the performance of a wireless or wired network.
SmartSniff - SmartSniff is a packet sniffer that capture TCP/IP packets and display them as sequence of conversations between clients and servers.