1. MongoDB (from "humongous") is a scalable, high-performance, open source NoSQL database.

  2. Redis is an open source in-memory data structure project implementing a distributed, in-memory key-value database with optional durability.

  3. Meet Neo4j: The graph database platform powering today's mission-critical enterprise applications, including artificial intelligence, fraud detection and recommendations.

  4. PostgreSQL is a powerful, open source object-relational database system.