If you are a serious web developer (and you happen to be working off a Windows platform), then chances are good that you currently have the excellent Fiddler web debugging proxy installed on your machine.
If you don’t, then perhaps you should consider rectifying this situation.
What Fiddler is at the heart of it all, is a simple web debugging proxy which basically logs all HTTP(S) traffic between your machine and the Internet. This allows you inspect all traffic, set breakpoints and basically fiddle with all outgoing and incoming data.
As a bonus, it comes with a powerful event-based scripting subsystem and can be extended using any .NET language – not to mention the multitude of add-ons already available for the system.
It is freeware, available for almost all version recent Windows versions and is able to debug traffic from virtually any application, including Internet Explorer, Firefox, Opera and Dropbox for example.
In other words, an invaluable tool in your web developer toolbox!
Related Link: http://www.fiddler2.com/fiddler2/