Firebase - Firebase is a cloud service designed to power real-time, collaborative applications for mobile and web.
Redis - Redis is an open source in-memory data structure project implementing a distributed, in-memory key-value database with optional durability.
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
neo4j - Meet Neo4j: The graph database platform powering today's mission-critical enterprise applications, including artificial intelligence, fraud detection and recommendations.
Socket.io - Realtime application framework (Node.JS server)
ArangoDB - A distributed open-source database with a flexible data model for documents, graphs, and key-values.