JsonAPI - Application and Data, Languages & Frameworks, and Query Languages
Blockbutler - Real-time transaction notifications for the blockchain
ReqRes - A hosted REST-API ready to respond to your AJAX requests.
Notif.me SDK - A Node.js lib to send all your transactional notifications
JSON Placeholder - JSON Placeholder is a modern platform that provides you online REST API, which you can instantly use whenever you need any fake data.
Pragma - The home base for Ethereum developers.