Blockbutler - Real-time transaction notifications for the blockchain
Telnyx SMS API - Build global text messaging right in your app
Notif.me SDK - A Node.js lib to send all your transactional notifications
Pragma - The home base for Ethereum developers.
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
Slack RTM API - Slack's Real Time Messaging API