Fiddler is billed as a "Web Debugging Proxy which logs all HTTP(S) traffic between your computer and the Internet"; it’s different from TCPView in the respect that monitoring traffic is only part of what it does. On top of network monitoring, it will also show every single file sent and received and if any new files have triggered network activity.
The best use of Fiddler other than for network debugging purposes is to look for malware – specifically with the IE browser (although it works in Firefox as well which I’ll describe in a moment). If you think there’s something IE is doing that you feel "just isn’t right", you can confirm whether your suspicions are correct by running Fiddler when IE is running. If there are any rogue programs accessing the network, Fiddler will show them. No, it won’t do anything about it since it’s just a monitor, but the point is that Fiddler can ‘see’ things you ordinarily couldn’t.
With the Firefox browser, Fiddler does install an extension called FiddlerHook:
This is normal and is required for Fiddler to work in Firefox. Once installed you will see it visible in the add-on bar at the bottom of the browser:
Tip: If you don’t see your add-on bar, this is how you enable it in Fx:
To enable network monitoring for Firefox through Fiddler, right-click and select "Use Fiddler automatically" or "Force traffic to Fiddler", and you’ll start seeing network monitoring results in the Fiddler program whenever you load anything from the internet in the Fx browser.
An added bonus is that you can clear your Fx cookies and cache direct from the right-click Fiddler menu.
Is Fiddler just for browsers?
No. Fiddler can monitor any network traffic just like TCPView can.
Where to get Fiddler: www.fiddler2.com
Is Fiddler free? Yes.