Wireshark – Network Protocol Analyzer
http://www.wireshark.org/

Same developers, same code, different name. The Ethereal network protocol analyzer has changed its name to Wireshark. Wireshark’s powerful features make it the tool of choice for network troubleshooting, protocol development, and education worldwide. Wireshark was written by an international group of networking experts, and is an example of the power of open source. It runs on Windows, Linux, UNIX, and other platforms. This has been added to World Wide Web Reference Subject Tracer™ Information Blog.