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/

Wireshark Packet Sniffer - formerly known as Ethereal - is a must-have tool for every Network Engineer, but these guys have got a cool OUI Lookup tool on their site as well.

Wireshark OUI Lookup Tool:
http://www.wireshark.org/tools/oui-lookup.html

Ethereal® is used by network professionals around the world for troubleshooting, analysis, software and protocol development, and education. It has all of the standard features you would expect in a protocol analyzer, and several features not seen in any other product. Its open source license allows talented experts in the networking community to add enhancements. It runs on all popular computing platforms, including Unix, Linux, and Windows.