Mockoon is the easiest and quickest way to design and run mock REST APIs.

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

Free & OpenSource REST API Testing Tool for all levels in a Test Pyramid

Swagger UI allows anyone — be it your development team or your end consumers — to visualize and interact with the API’s resources without having any of the implementation logic in place

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

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.