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Mockoon is the easiest and quickest way to design and run mock REST APIs. No remote deployment, no account required, free and open-source.
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WireMock - a web service test double for all occasions.
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Testerum is a Free & Open source test automation framework for Web Apps and REST APIs. It integrates Selenium, HTTP, Mock 3rd Party API, databases, etc. Testerum comes with a friendly UI where people can write automated tests without coding.