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

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.

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

Robotium is an Android test automation framework that has full support for native and hybrid applications.

Real API Design-first Development Platform Design. Debug. Test. Document. Mock. Build APIs Faster & Together.

Built for developers and testers alike, SoapUI is the only open source tool that covers the entire testing spectrum (functional, security, load, mocking).