JsonAPI - Application and Data, Languages & Frameworks, and Query Languages
Notif.me SDK - A Node.js lib to send all your transactional notifications
ReqRes - A hosted REST-API ready to respond to your AJAX requests.
Pusher Beams - Send push notifications programmatically at scale
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.
Instapush - Instant notifications for transactions that matter.