Socket.io - Realtime application framework (Node.JS server)
Firebase - Firebase is a cloud service designed to power real-time, collaborative applications for mobile and web.
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
Supabase - An open source Firebase alternative
Ably - Serious realtime infrastructure
neo4j - Meet Neo4j: The graph database platform powering today's mission-critical enterprise applications, including artificial intelligence, fraud detection and recommendations.