WireShark is a free network protocol analyzer for Unix and Windows. It allows you to examine data from a live network or from a capture file on disk. You can interactively browse the capture data, viewing summary and detail information for each packet. Ethereal has several powerful features, including a rich display filter language and the ability to view the reconstructed stream of a TCP session.

In terms of the ECF, on a practical level Wireshark has been used in the configuration of payement gateways to detect issues such as certificate errors. 

You can learn about Wireshark in their FAQ 

Wireshark's site