Fiddler is a Web 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 includes a powerful event-based scripting subsystem, and can be extended using any .NET language.
Fiddler is freeware and can debug traffic from virtually any application, including Internet Explorer, Mozilla Firefox, Opera, and thousands more.
Fiddler 2 has replaced Fiddler 1.x. Fiddler 2 supports debugging HTTPS traffic, a richer extensibility model, and can by installed side-by-side with Fiddler 1.x if desired.