WireShark

Wireshark (previously known as Ethereal) is a open source network protocol analyzer that runs on both Unix and Windows.
Wireshark is used to capture or to examine data live on a network .

Features:

• Rrich display filter language
• Ability to view the reconstructed stream of a TCP session
• See network data and packages live.
• Capture network traffic and save to a file
• supports hundreds of protocols and media types.
• A tcpdump-like console version named tethereal