Wireshark

Wireshark is the world's foremost network protocol analyzer, and is the standard in many industries.

It is the continuation of a project that started in 1998. Hundreds of developers around the world have contributed to it, and it is still under active development.

Features
Wireshark has a rich feature set which includes the following:

* Hundreds of protocols are supported, with more being added all the time
* Live capture and offline analysis are supported
* Standard three-pane packet browser
* Multi-platform: Runs on Windows, Linux, OS X, Solaris, FreeBSD, NetBSD, and many others
* Captured network data can be browsed via a GUI, or via the TTY-mode TShark utility
* The most powerful display filters in the industry
* Rich VoIP analysis
* Read/write many different capture file formats: tcpdump (libpcap), Catapult DCT2000, Cisco Secure IDS iplog, Microsoft Network Monitor, Network General Sniffer