The Apache JMeter™ application is open source software, a 100% pure Java application designed to load test functional behavior and measure performance. It was originally designed for testing Web Applications but has since expanded to other test functions.

Mock and modify HTTP request for exploratory testing.

Robot Framework is a generic open source automation framework. It can be used for test automation and robotic process automation (RPA).

Professional software tool for HTTP testing and debugging

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.

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