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.
cc3b61764681686be8c3e715adb04a29ac05b47f6bebb64b9583a9ec4a088a62