Here's a great Microsoft tool that deserves more attention than it gets. Fiddler is a free debugger software that hooks onto your Internet Explorer or Firefox browser. It allows a programmer to see the HTTP packets passing through the machine in a nicely arranged way.
I used to run Proxomitron, another amazing proxy software written by Scott Lemmon who sadly retired the software in 2003. Although the software was free, it wasn't open source (actually he termed it as "shannon-ware" because he was in love with a japanese rock band.). It was a great software because you could create scripts to filter out ads or optimize a HTML page on the fly. More importantly, it allowed me to view the HTTP headers passing through a browser. I pretty much mastered the entire HTTP protocol reading from the black and green screen in Proxomitron (W3C helped too some times).
I saw new hope when I discovered Fiddler. The download is located at a very plain site, almost homepage look-alike, that you wouldn't guess it's produced by Microsoft. The license agreement does contain the usual Microsoft disclaimer -- just to be sure. If you get Fiddler2 (version 2.x, currently in Beta but very stable), you can even debug HTTPS web sites.
Fiddler also comes with filters, performance stats, session inspector and even allows you to handcraft your HTTP headers. It beats Proxomitron hands down for debugging!
Click here to download Fiddler (requires .NET 2.0 runtime)