Wireshark is a network protocol analyzer, and is the standard in many industries.
It is the continuation of a project that started in 1998. Hundreds of developers around the world have contributed to it, and it is still under active development. Read or write many different capture file formats: tcpdump (libpcap), Catapult DCT2000, Cisco Secure IDS iplog, Microsoft Network Monitor, NAI Sniffer (compressed and uncompressed), Sniffer Pro, and NetXray, Network Instruments Observer, Novell LANalyzer, RADCOM WAN or LAN Analyzer, Shomiti or Finisar Surveyor, Tektronix K12xx, Visual Networks Visual UpTime, and WildPackets EtherPeek,TokenPeek, or AiroPeek.