emitter.io - Real-time communication service for connecting online devices.
Socket.io - Realtime application framework (Node.JS server)
OneSignal - Customer engagement platform used by over 1 million developers and marketers; the fastest and most reliable way to send mobile and web push notifications, in-app messages, emails, and SMS.
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
NATS - NATS.io is an open source messaging system for cloud native applications, IoT messaging, Edge, and microservices architectures.
Rapid.io - Rapid.io is a cloud-hosted service that allows app developers to build realtime user interfaces...