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.

CrossBrowserTesting - Browser Testing made simple! Run automated, visual, and manual tests on 1500+ real browsers and mobile devices. Test more browsers, in less time.

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.

Codeception - Codeception is a new full-stack testing PHP framework.

Retest - Retest allows you to check your web apps user experience and deliver absolute masterpieces on multiple devices so that every time your customer visits your webpage, they feel what you want them to feel with your design irrespective of their device

TestingWhiz - TestingWhiz is a codeless test automation tool for software, mobile and web.