Mockable - Quickly define some REST or SOAP endpoints and have them return json/xml (dummy) data.
MockServer - Easy mocking of any system you integrate with via HTTP or HTTPS.
Postman - The Collaboration Platform for API Development
Request inspector - Debug web hooks, http clients
CurlHub.io - API Traffic Inspector
WireMock Cloud - Fast and simple API mocking in the cloud, powered by WireMock.