k6 Open Source is designed for load testing. Extensible to support other types of testing.

Streamline your API testing, integration testing and test data management.

Design, debug, and test APIs locally or in the cloud

Postman is an API platform for building and using APIs. Postman simplifies each step of the API lifecycle and streamlines collaboration so you can create better APIs—faster.

An open source JS tool for image analysis and comparison. A successor of PhantomCSS.

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.