Beeceptor - Unblock yourself from API dependencies, and build & integrate with APIs fast. Beeceptor helps you build a mock Rest API in a few seconds.
soapUI - SoapUI Pro is one of the most prominent API testing platforms around, allowing developers to quickly prototype the functions of their apps and get them to market with little hassle.
WireMock - WireMock - a web service test double for all occasions.
Postman - The Collaboration Platform for API Development
Mockable - Quickly define some REST or SOAP endpoints and have them return json/xml (dummy) data.
Runscope - Log, monitor and measure your API usage to solve API problems fast.