Wireshark (formerly 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.

It has many features related VoIP, including SIP, H323, and RTP stream analysis and playback.

Main site: http://www.wireshark.org/
Wiki: http://wiki.wireshark.org/