A free packet sniffer....
Wireshark is a free packet sniffer computer application. It is used for network troubleshooting, analysis, software and communications protocol development, and education.
Wireshark, the most popular network protocol analysis tool has finally turned to version 1.0.
This released version is not much different than the previous version 99.8 released in Feb 27. Apart from some cosmetic improvement from the previous version this version includes an experimental Mac OS X package.
You can download wireshark 1.0.0 from here
For a more complete list of information about this release, read the official release note here.