Mockable - Quickly define some REST or SOAP endpoints and have them return json/xml (dummy) data.
API List - A collective list of APIs. Build something.
Postman - The Collaboration Platform for API Development
Dash 4 - Instant offline access to 150+ API documentation sets
MockServer - Easy mocking of any system you integrate with via HTTP or HTTPS.
Phantombuster - A marketplace of simple to use no-code APIs