Mocky.io

Mocky is really simple, very easy to use, even for users with no prior experience with API mocking. To create a mock API, simply enter the URL, HTTP method, headers, and body of the request, and then enter the desired response. It can also specify delays and errors if needed. It doesn't require sign-up or login, which is really nice feature! The API can also be set to expiry to guarantee secrecy. Mocky also supports multiple response formats, from JSON, XML, simple text, etc.

Best open source tool for mocking your APIs