Tools for debugging web applications

Fiddler. This program runs as a proxy that logs all HTTP traffic. It supports Internet Explorer and other browsers. Using Fiddler, you can examine each request and response, including headers, cookies, and the message contents.

Web Development Helper. This tool works only with Internet Explorer, but in addition to logging HTTP traffic, it can be used to inspect the HTML DOM, display trace information in a separate window, shut down the application, and decode the page's view state.

"Atlas" is amazing...

Microsoft introduce "Atlas," powerful new framework for building cross–browser, cross–platform AJAX applications in its MIX06 event. Compared to Google, Yahoo, Microsoft is more focusing on the developement tools instead of the applications for Web 2.0. Now this baby is really a powerful tools for asp.net 2.0.