Fiddler Web Debugging Proxy
Went to an MSDN event by Russ (http://blogs.msdn.com/rfustino/) a few weeks ago and during his demo he displayed this cool tool called Fiddler (http://www.fiddler2.com/). A must have debugging tool for any Smart Client or Web Developer.
Fiddler is a Web Debugging Proxy which logs all HTTP(S) traffic between your computer and the Internet. Fiddler allows you to inspect all HTTP(S) 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.