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.
41665c8dc7ea0746a504ecf19cc4a77f307fed8ffc8c1d1bac3d4b2a19710780