Wireshark

http://www.wireshark.org/download.html

Wireshark® is the world's most popular network protocol analyzer. It has a rich and powerful feature set and runs on most computing platforms including Windows, OS X, Linux, and UNIX. Network professionals, security experts, developers, and educators around the world use it regularly. It is freely available as open source, and is released under the GNU General Public License version 2.
It is developed and maintained by a global team of protocol experts, and it is an example of a disruptive technology.