Wireshark - Open Source Tools for System Administrators
Known as Ethereal prior to 2006, Wireshark is one of the most powerful network tools available today. The software captures and analyzes packets from live Ethernet, IEEE 802.11, PPP, raw USB and loopback connections and is especially useful in network troubleshooting. You can customize the display, create plug-ins to support new protocols via a display filter and captured files can be edited or converted via command-line input.
Supported OS: BSD, Linux,Mac OS X, Windows, UNIX