Selenium - Selenium automates browsers. That's it! What you do with that power is entirely up to you. Primarily, it is for automating web applications for testing purposes, but is certainly not limited to just that.
Fake - Fake is a new browser for Mac OS X that makes web automation simple.
Sauce Labs - Test mobile or web apps instantly across 700+ browser/OS/device platform combinations - without infrastructure setup.
Watir - Watir stands for Web Application Testing In Ruby. It facilitates the writing of automated tests by mimicking the behavior of a user interacting with a website. CommunityWatir stands for Web Application Testing In Ruby.
Browsershots - Browsershots makes screenshots of your web design in different browsers.
IBM Rational Functional Tester - IBM Rational Functional Tester provides automated testing capabilities for functional, regression, GUI and data-driven testing.