Firebase - Firebase is a cloud service designed to power real-time, collaborative applications for mobile and web.
Supabase - An open source Firebase alternative
restdb.io - restdb.io lets you create databases and REST APIs quickly. Development databases are free. Powerful web-based and mobile-friendly data management.
neo4j - Meet Neo4j: The graph database platform powering today's mission-critical enterprise applications, including artificial intelligence, fraud detection and recommendations.
RemoteStorage - An open protocol for per-user storage OWN YOUR DATA
Redis - Redis is an open source in-memory data structure project implementing a distributed, in-memory key-value database with optional durability.