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