Voiden is a developer-first API client with local storage, Git-integrated, all based on markdown + offline support.

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

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

Hoppscotch is an open-source API development ecosystem that helps you create and test your API requests saving precious time in development.

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.

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.