Voiden is a developer-first API client with local storage, Git-integrated, all based on markdown + offline support.

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

Fully integrated API and web service testing capabilities that automate end-to-end functional API testing.

Jest is a delightful JavaScript Testing Framework with a focus on simplicity.

k6 Open Source is designed for load testing. Extensible to support other types of testing.

Testing and validating REST services in Java is harder than in dynamic languages such as Ruby and Groovy. REST Assured brings the simplicity of using these languages into the Java domain.