Built for developers and testers alike, SoapUI is the only open source tool that covers the entire testing spectrum (functional, security, load, mocking).

Browser and Mobile Automation Test Framework for Node.js

Test, scrape, generate, and automate without the pain and expense of hosting your own headless browsers.

Percy is a visual testing and monitoring tool that focuses on detecting visual changes in web applications

Bruno is a Fast and Git-Friendly Opensource API client, aimed at revolutionizing the status quo represented by Postman, Insomnia and similar tools out there.

Burp or Burp Suite is a set of tools used for penetration testing of web applications. It is developed by a company named Portswigger who make freely available a Web Security Academy which has a range of lessons on penetration testing.