In my quest to find better ways to debug and understand what's going on with various sites I've tried a lot of tools and techniques, but at the end of the day it's the simple ones you keep coming back to.
Fiddler is one of those really simple but powerful apps that fills a niche.
Want to know what's really going on when your browser talks to the server? Care about the headers, the cookies, every byte in the request, the number of connections - all of which are important when debugging and performance tuning.
Curious about what information your server is sharing without you knowing - vital when trying to lock a site down and not give crackers too much information that you don't have to share.
Fiddler makes that job a lot easier.
It's an 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.
Fiddler is freeware and can debug traffic from virtually any application, including Internet Explorer, Mozilla Firefox, Opera, and thousands more.