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: Run... More info »
Great application. It's a bit much for mere mortals but that doesn't really matter.
