What is the best way to test my webforms applications?
Looks like people are loving Watin, and selenium.
Here is a review of Watin,Watir and Selenium http://adamesterline.com/2007/04/23/watin-watir-and-selenium-reviewed/
Apparently Selenium worked quite slow for the tester but if you'll notice, as one of the comments points out, that this is only the case due to its support of multiple browsers.
However there is a CTP (Community Technology Preview) release of WatiN which offers support for both Internet Explorer and FireFox automation.
That's the biggest shortcoming of Webforms -- it's, for all practical reasons, untestable in terms of unit testing of testing controllers, etc.
That is one of the major advantages of the MVC framework.