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
Pusher Beams - Send push notifications programmatically at scale
CryptoNotify - Receive transaction notifications of any BTC or ETH address
Pragma - The home base for Ethereum developers.