Google Open Source Blog: Introducing WebDriver
http://google-opensource.blogspot.com/2009/05/introducing-webdriver.html

Google offers a new tool which helps out with testing web apps using Firefox and IE extensions rather than pure JS.

Works as an extension in Firefox, uses Automation controls in IE etc.

"Selenium 2.0 will offer WebDriver's API alongside the traditional Selenium API, and we shall be merging the two implementations to offer a capable, flexible testing framework"

WebDriver is a clean, fast framework for automated testing of webapps.