JsonAPI - Application and Data, Languages & Frameworks, and Query Languages
Pragma - The home base for Ethereum developers.
ReqRes - A hosted REST-API ready to respond to your AJAX requests.
Blockbutler - Real-time transaction notifications for the blockchain
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.
CryptoNotify - Receive transaction notifications of any BTC or ETH address