Pusher Beams - Send push notifications programmatically at scale
Ethereum Push Notification Service - Protocol for incentivized, blockchain based notifications.
Mentioned By - Learn when your site is mentioned by other sites for free
Blockbutler - Real-time transaction notifications for the blockchain
Now Push Notify - iOS does not support browser notifications today, but Now Push Notify makes this possible.
Notif.me SDK - A Node.js lib to send all your transactional notifications