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.

Checks web UI for browser compatibility (Compliance with Can I Use?)

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

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.

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

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