WireMock.Net
WireMock.Net is a product for stubbing and mocking web HTTP responses
...It's easy to launch a mock API server and simulate a host of real-world scenarios and APIs - including REST, SOAP, OAuth2 and more. WireMock is a free API mocking tool that can be run as a standalone server, or in a hosted version via the WireMock Cloud-managed service. API mocking involves creating a simple simulation of an API, accepting the same types of request and returning identically structured responses as the real thing, enabling fast and reliable development and testing.