Ethereal, Open Source Network Sniffer
Ethereal is a free network protocol analyzer for Unix and Windows. It allows you to examine data from a live network or from a capture file on disk. You can interactively browse the capture data, viewing summary and detail information for each packet.
Ethereal has several powerful features, including a rich display filter language and the ability to view the reconstructed stream of a TCP session. A text-based version called tethereal is included.
URL: http://www.wireshark.org/
