wireshark

Wireshark is the world's foremost network protocol analyzer. It lets you capture and interactively browse the traffic running on a computer network. It is the de facto (and often de jure) standard across many industries and educational institutions.
Wireshark development thrives thanks to the contributions of networking experts across the globe. It is the continuation of a project that started in 1998.
Features

Wireshark has a rich feature set which includes the following:

• Deep inspection of hundreds of protocols, with more being added all the time

• Live capture and offline analysis

• Standard three-pane packet browser

• Multi-platform: Runs on Windows, Linux, OS X, Solaris, FreeBSD, NetBSD, and many others

• Captured network data can be browsed via a GUI, or via the TTY-mode TShark utility

• The most powerful display filters in the industry

• Rich VoIP analysis

• Read/write many different capture file formats: tcpdump (libpcap), Pcap NG, Catapult DCT2000, Cisco Secure IDS iplog, Microsoft Network Monitor, Network General Sniffer® (compressed and uncompressed), Sniffer® Pro, and NetXray®, Network Instruments Observer, NetScreen snoop, Novell LANalyzer, RADCOM WAN/LAN Analyzer, Shomiti/Finisar Surveyor, Tektronix K12xx, Visual Networks Visual UpTime, WildPackets EtherPeek/TokenPeek/AiroPeek, and many others

• Capture files compressed with gzip can be decompressed on the fly

• Live data can be read from Ethernet, IEEE 802.11, PPP/HDLC, ATM, Bluetooth, USB, Token Ring, Frame Relay, FDDI, and others (depending on your platform)

• Decryption support for many protocols, including IPsec, ISAKMP, Kerberos, SNMPv3, SSL/TLS, WEP, and WPA/WPA2

• Coloring rules can be applied to the packet list for quick, intuitive analysis

• Output can be exported to XML, PostScript®, CSV, or plain text

You can download it from HERE