Ethereal 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 Ethereal features that are missing from closed-source sniffers. Screenshot available here.
ea6ddc17081ac84412c8a67a69ab99933e498b3825ed28dd5c0c4ccb78237ec9