Wireshark/Ethereal is a free network protocol analyzer for almost all operating systems (including Unix, Linux and MS Windows). It allows you to examine data from a live network or from a capture file on disk. You can interactively browse the capture data, viewing summary and detail information for each packet. Wireshark/Ethereal has several powerful features, including a rich display filter language and the ability to view the reconstructed stream of a TCP session.
The installation of Wireshark is easy so I will not mention here, you can find newest Wireshark version at http://www.wireshark.org/download.html