Wireshark (Formely Ethereal)

Wireshark is a GTK+-based network protocol analyzer, or sniffer, that lets you
capture and interactively browse the contents of network frames. The goal of
the project is to create a commercial-quality analyzer for Unix and to give
Wireshark features that are missing from closed-source sniffers.

Works great on both Linux and Windows (with a GUI), easy to use and can
reconstruct TCP/IP Streams! Will do a tutorial on Wireshark later.

Get Wireshark Here - http://www.wireshark.org/