Wireshark 1.4

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 deep inspection of hundreds of protocols, with more being added all the time, live capture and offline analysis, standard three-pane packet browser, the most powerful display filters in the industry Rich VoIP analysis, support for many different capture file formats, decryption support for many protocols, and much more.