We have released a new version of fiddler and dramatically improved the web test generation from a fiddler recording. Get it at www.fiddler2.com/fiddler2.
What is fiddler? Fiddler is an http proxy debugging tool that allows you to record over an http proxy. This complements the VSTS web test recorder, which hooks IE events to capture http traffic. The VSTS 2005 web test recorder has a couple of limitations: 1) it does not record AJAX requests, and 2) it does not record requests from DoModalPopup calls (but one way to do popups in IE, other ways are recorded correctly (such as using the Target attribute on a link).
The first version of fiddler didn't do all the great fixups that the VS recorder does: filter redirects, filter dependents, and automatically correlate viewstate. The new version does, and has a powerful design that lets you write custom recorder plugins that you can use for a variety of things, the most common being dynamic parameter correlation like the viewstate problem. See Sean's blog posts for more info:
This new fiddler also lets you record over https, a limitation of the previous version of fiddler.