Wireshark is a free tool for monitoring network traffic.
The wireshark download is at
http://www.wireshark.org/download.html - there are downloads for Windows and several Unix platforms.
At
http://www.youtube.com/watch?v=qevSDkiW9z8&feature=share, Pluralsite have provided a short training video.
There is an good introduction to WireShark at
http://en.wikipedia.org/wiki/WiresharkHere is a screenshot courtesy of Wikipedia.
Learn about addressing network performance management issues and
enhancing your Wireshark experience by perusing a complimentary copy of Network Monitoring and Troubleshooting For Dummies. (or at http://media.wiley.com/assets/7005/17/9781118217399_custom.pdf)
More information at http://www.wireshark.org/docs/