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.

Some people have heard of this and some have not but I hope you enjoy.


Can also get more questions asked on that site.