Blockbutler - Real-time transaction notifications for the blockchain
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Notif.me SDK - A Node.js lib to send all your transactional notifications
Ultimate Push Notifications by Ecomfit - Retarget customers and protect abandoned cart to boost sale
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
Ethereum Push Notification Service - Protocol for incentivized, blockchain based notifications.