Professional software tool for HTTP testing and debugging

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

REST API client library for .NET

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

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.

Hand-crafted lightweight Rest Client for testing APIs. Supports collections, environments, git collaboration and local storage.