Blockbutler - Real-time transaction notifications for the blockchain
Notif.me SDK - A Node.js lib to send all your transactional notifications
Ethereum Push Notification Service - Protocol for incentivized, blockchain based notifications.
Pragma - The home base for Ethereum developers.
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
Clank - Explore the biggest crypto transactions across blockchains