<p>What is the best way to test my webforms applications?</p>

<p>Looks like people are loving Watin, and selenium.</p>

UPDATE
:
Given WatiN has been stagnant for over a year now, I would direct anyone that needs web ui tests towards
selenium
, it is in continuous use & development by many contributors, and is actively used by Google.
WatiN
is the best that I've found.  It integrates into Visual Studio unit testing or nunit & you can do pretty much anything you need in the browser (click links, submit forms, look for text/images, etc.)
See the following questions for similar answers:
What is the best way to do unit testing for ASP web pages (C#)?
Web Application Testing for .Net (watin Test Recorder)
How do you programmatically fill in a form and ‘POST’ a web page?