Fiddler on the...uh...Browser Roof

Fiddler, as the site explains, is “a 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 is designed to be much simpler than using NetMon or Achilles, and includes a simple but powerful JScript.NET event-based scripting subsystem.”

http://www.fiddlertool.com/fiddler/

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Here’s an excerpt from the introduction to Fiddler on MSDN:

Have you ever found yourself wondering how Microsoft Internet Explorer interacts with your Web application? Have you encountered a strange performance bottleneck that you can't track down? Are you curious about which cookies are being sent, or what downloaded content is marked as cacheable?

Microsoft Fiddler can help you answer these questions, and many more. Fiddler is an HTTP debugging proxy that logs all HTTP traffic between your computer and the Internet. Fiddler enables you to inspect all HTTP traffic, set breakpoints, and "fiddle" with incoming or outgoing data. Fiddler is much simpler to use than NetMon or other network debuggers because it exposes only HTTP traffic and does so in a user-friendly format.

Fiddler includes a simple but powerful Microsoft JScript .NET event-based scripting subsystem flexible enough to support a broad array of HTTP debugging tasks. Written in C# on the Microsoft .NET Framework, Fiddler is available as an unsupported PowerToy for Internet Explorer.