Wireshark

Wireshark is a quality network protocol analysis tool with a rich set of features:

• In-depth inspection of hundreds of protocols
• offline analysis
• capture and detailed examination of data flowing through networks
• improved VoIP communications analysis
• advanced data filter
• live data reads from Ethernet, IEEE 802.11, PPP/HDLC, ATM, Bluetooth, USB, Token Ring, Frame Relay, FDDI, ...
• multi-platform: runs on Windows, Linux, OS X, Solaris, FreeBSD, NetBSD, and many more
• graphical user interface.