Fiddler is a free 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 includes a powerful event-based scripting subsystem, and can be extended using any .NET language.
The most primitive use of Fiddler is to view the contents of single request and response, though this basic ability proves useful when developing rich web functionality with AJAX — debugging problems or just trying to get a full picture of the AJAX traffic.
Fiddler 2 requires version 2.0 of the .NET Framework. There is also a deprecated v1.3 for developers still using the.NET Framework 1.1. Fiddler will auto-configure the debugging proxy for IE6 and IE7, but the proxy is compatible with Firefox, Opera and virtually any application using the HTTP(S) protocol. For example, Firefox allows manually specifying a proxy through Menu > Preferences > Network > Settings > Manual Proxy Configuration: 127.0.0.1 Port 8888.