Mockable - Quickly define some REST or SOAP endpoints and have them return json/xml (dummy) data.
PublicAPIs - Explore the largest API directory in the galaxy
MockServer - Easy mocking of any system you integrate with via HTTP or HTTPS.
The Unsplash API - No-limits, do-what-you-want API for access to 200K+ HD pics.
Postman - The Collaboration Platform for API Development
API List - A collective list of APIs. Build something.