Selenium is what I'm using, Selenium IDE (firefox plugin) makes it easy to set up tests. http://www.seleniumhq.org

UserTesting.com is great for usability testing, they have people follow some of your instructions and record their screen/voice as they go through it. Feedback from this has lead to a lot of critical improvements.

Another article you might be interested in is about how software is developed for the space shuttle (they test like crazy). Someone posted the link on HN last night and it's fascinating: http://www.fastcompany.com/magazine/06/writestuff.html