emitter.io - Real-time communication service for connecting online devices.
simperium - Move data everywhere it's needed, instantly and automatically
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.
GUN - Self-hosted Firebase.
NATS - NATS.io is an open source messaging system for cloud native applications, IoT messaging, Edge, and microservices architectures.
PubNub - PubNub is a real-time messaging system for web and mobile apps that can handle API for all platforms and push messages to any device anywhere in the world in a fraction of a second without having to worry about proxies, firewalls or mobile drop-offs.