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.

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.

Loader.io is a free load testing service that allows you to stress test your web-apps & apis with thousands of concurrent connections.

Simplifies performance and load testing so that application delivery teams can take a standardized approach to ensure continuous performance, reliability, and scalability from development to production.

ReadyAPI is a low-code API testing platform for development teams that care about creating test automation across any workflow.