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.
Applitools Eyes - Automated visual application testing and monitoring
DalekJS - DalekJS is an open source UI testing tool.
VisualReview - VisualReview, a human-friendly tool for testing and reviewing visual regressions.
Webinject - WebInject is a free tool for automated testing of web applications and web services.
Imagium.io - Accelerate your software testing process with one of the most advanced visual comparison tool from the future