OrientDB - OrientDB - The World's First Distributed Multi-Model NoSQL Database with a Graph Database Engine.
MongoDB - MongoDB (from "humongous") is a scalable, high-performance NoSQL database.
PostgreSQL - PostgreSQL is a powerful, open source object-relational database system.
Redis - Redis is an open source in-memory data structure project implementing a distributed, in-memory key-value database with optional durability.
ArangoDB - A distributed open-source database with a flexible data model for documents, graphs, and key-values.
RethinkDB - The open-source database for the realtime web