Windmill!

Selenium is a suite of tools specifically for testing web applications. Selenium IDE is a Firefox add-on that records clicks, typing, and other actions to make a test, which you can play back in the browser. Selenium Remote Control (RC) runs your tests in multiple browsers and platforms. Tweak your tests in your preferred language.

Watir, pronounced water (acronym standing for "Web Application Testing in Ruby"), is an open-source (BSD) family of Ruby libraries for automating web browsers. It allows you to write tests that are easy to read and maintain. It is simple and flexible.

PhearJS renders dynamic webpages using PhantomJS: fetch a page, render it and return a pretty JSON object. PhearJS is offered as a hosted API as well as an open source solution that you can host yourself.

FunkLoad is a functional and load web tester, written in Python, whose main use cases are:

Screenster provides visual UI test automation for web applications. It is the only tool that validates the screens that users actually see.