Blockbutler - Real-time transaction notifications for the blockchain
Notif.me SDK - A Node.js lib to send all your transactional notifications
Pragma - The home base for Ethereum developers.
ZeroPush - Simple Push Notifications for Developers
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
The Simple Postcard - Text a photo to mail it as a postcard