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
Pragma - The home base for Ethereum developers.
Instapush - Instant notifications for transactions that matter.
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.