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

Puppeteer is a Node.js library which provides a high-level API to control Chrome/Chromium over the DevTools Protocol.

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.

API Testing Platform that automatically generates unit test cases with dependency mocks from API calls

REST API client library for .NET

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