Selenium

Really really neat work Jason Huggins did on his Selenium Project.
He recently has become a Noogler(new people to Google) now.
An interview has been done and posted in Google Testing Blog about what he did and why he did it.

http://www.openqa.org/selenium/
Selenium tests run
directly in a browser, just as real users do. And they run in
Internet Explorer, Mozilla and Firefox on Windows, Linux, and Macintosh. No
other test tool covers such a wide array of platforms.