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 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