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.
DalekJS - DalekJS is an open source UI testing tool.
Webinject - WebInject is a free tool for automated testing of web applications and web services.
UI.Vision - Modern open-source task and test automation tool and Selenium IDE.
Pincers - Web Automation API that runs on top of WebDriver
Katalon - Built on the top of Selenium and Appium, Katalon Studio is a free and powerful automated testing tool for web testing, mobile testing, and API testing.