API List - A collective list of APIs. Build something.
Postman - The Collaboration Platform for API Development
Abstract APIs - Simple, powerful APIs for everyday dev tasks
Firefox Developer Tools - Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile.
The Unsplash API - No-limits, do-what-you-want API for access to 200K+ HD pics.
Cocoa Rest Client - A free open source, native Apple OS X app for testing HTTP/REST endpoints