Blockbutler - Real-time transaction notifications for the blockchain
Notif.me SDK - A Node.js lib to send all your transactional notifications
Pushover - Real-time notifications on your Android, iPhone, iPad, and Desktop
Ethereum Push Notification Service - Protocol for incentivized, blockchain based notifications.
Instapush - Instant notifications for transactions that matter.
Pusher Beams - Send push notifications programmatically at scale