http://www.fiddlertool.comFound this link today on a blog. This is a power toy from Microsoft that sets up a proxy between your site and your browser to monitor taffic between them. This gives you the full details of the data being communicated.
You can set breakpoints and monitor both incoming and outgoing date between your site and your browser. View headers, response codes, cookies, etc.
You can even extend Fiddler with .NET coding.