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

A tool to help create test data via a UI or API

REST API client library for .NET

Bruno is a Fast and Git-Friendly Opensource API client, aimed at revolutionizing the status quo represented by Postman, Insomnia and similar tools out there.

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

Unifying API testing, API Performance testing, API Mocks and UI testing.