Mocklets - A HTTP-based API simulator to assist parallel development 🛠️
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
Airbnb Official API - Develop your application for millions of travelers & hosts
MockServer - Easy mocking of any system you integrate with via HTTP or HTTPS.