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.
Scapy - Scapy is a powerful interactive packet manipulation program.
tcpdump - tcpdump is a common packet analyzer that runs under the command line.
Nipper - Nipper - Neat Internet Protocol Packet EditoR - is a graphical packet generation tool.
Ettercap - Ettercap is a suite for man in the middle attacks on LAN.
packeth - packeth is GUI and CLI packet generator tool for ethernet.