Mockoon - Mockoon is the easiest and quickest way to design and run mock REST APIs. No remote deployment, no account required, free and open-source.

Beeceptor - Unblock yourself from API dependencies, and build & integrate with APIs fast. Beeceptor helps you build a mock Rest API in a few seconds.

WireMock - WireMock - a web service test double for all occasions.

SOAP/REST Mock Service - Download SOAP/REST Mock Service for free.

MockServer - Easy mocking of any system you integrate with via HTTP or HTTPS.

Mockintosh - Mocks for microservice environments