Another notable tool for developers is "Fiddler".  

What is Fiddler?

Fiddler is a HTTP Debugging Proxy which logs all HTTP traffic between your computer and the Internet. Fiddler allows you to inspect all HTTP Traffic, set breakpoints, and "fiddle" with incoming or outgoing data. Fiddler is designed to be much simpler than using NetMon or Achilles, and includes a simple but powerful JScript.NET event-based scripting subsystem.

What can it be used for?

In terms of the ECF, if you need to see data that is sent out or coming in, you can use Fiddler to view this traffic.  One particular instance would be when you are working with shipping or payment gateways, don't have fullsource, and wish to investigate traffic coming and going in order to troubleshoot. 

More information

Introduction to Fiddler on MSDN

Download

Fiddler Tool