Blockbutler - Real-time transaction notifications for the blockchain
Mentioned By - Learn when your site is mentioned by other sites for free
CryptoNotify - Receive transaction notifications of any BTC or ETH address
Ultimate Push Notifications by Ecomfit - Retarget customers and protect abandoned cart to boost sale
Notif.me SDK - A Node.js lib to send all your transactional notifications
Ethereum Push Notification Service - Protocol for incentivized, blockchain based notifications.