Fiddler allows you to
inspect 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 that supports a proxy,
including Internet Explorer, Google Chrome, Apple Safari, Mozilla Firefox, Opera, and thousands more.
You can also debug traffic from popular devices like
Windows Phone, iPod/iPad, and others.