Wireshark

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!.

Download the zip