By Telerik about Fiddler
Fiddler is a free Web Debugging Proxy that works seamlessly across most platforms, systems and browsers. It logs all incoming and outgoing traffic from your computer to the Internet and allows you to debug traffic from practically any application that supports a proxy.
It is possible to manipulate and edit web sessions using Fiddler by simply setting a breakpoint so that the session process can be paused, thereby giving you the opportunity to alter the request or response. As well as Web session manipulation, you can also create your own HTTP requests that can run through Fiddler allowing you to reproduce complex scenarios which may otherwise be really tricky to test.
We all know that security is really important, so you'll be pleased to know that Fiddler has this covered as well. You can use it to test your web applications by decrypting either all HTTPS traffic or specific sessions. Further to this you can also make sure that all cookies, headers and cache directives that are being transferred between the server and the client are all correct.
There are a plethora of ready-made add-ons at your disposal, making Fiddler highly customizable and flexible to suit your needs. However it is worth noting that although the interface looks simple, a lot of functions and tools are available therefore Fiddler is better suited to power users, developers, testers and system administrators.
Please note this download is for users of .NET 4, however a version is also available for users of .NET 2.