Design, debug, and test APIs locally or in the cloud

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

Build and test APIs easily

REST API client library for .NET

WebLOAD – Performance and Load Testing for the Most Challenging Web Applications

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.