Jest is a delightful JavaScript Testing Framework with a focus on simplicity.

A browser extension and Ruby gem to improve the debugging experience when testing Rails applications

Requestly is an open-source tool that simplifies API development, testing, mocking, and integration for developers and QAs.

Next-gen browser to build, test & debug mobile websites.

Sonic is a platform that integrates remote control debugging and automated testing of mobile devices, and strives to create a better user experience for global developers and test engineers.

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.