netcat - Netcat is a featured networking utility which reads and writes data across network connections...
Ixchariot - Instant performance assessment of complex networks from pre- to post-deployment.
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
SmartSniff - SmartSniff is a packet sniffer that capture TCP/IP packets and display them as sequence of conversations between clients and servers.
Ostinato - Ostinato is a packet crafter and traffic generator for network engineers. Wireshark in Reverse.