Blockbutler - Real-time transaction notifications for the blockchain
Pusher Beams - Send push notifications programmatically at scale
Notif.me SDK - A Node.js lib to send all your transactional notifications
Mentioned By - Learn when your site is mentioned by other sites for free
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
Now Push Notify - iOS does not support browser notifications today, but Now Push Notify makes this possible.