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Fiddler is an extensible web-debugging platform which can be extended with any .NET language. It allows you to write pluggable inspectors to display traffic in a context-sensitive manner, and allows tampering with any request or response before it's sent or returned to the application. It has an "autoresponse" mechanism that allows you to replay previously captured or generated traffic, and can archive traffic to disk in a compressed, lossless format for later investigation.
See http://www.fiddler2.com/fiddler/help/video/ for some quick tutorials that show some of the features of Fiddler.