MongoDB - MongoDB (from "humongous") is a scalable, high-performance NoSQL database.
RethinkDB - The open-source database for the realtime web
CouchDB - HTTP + JSON document database with Map Reduce views and peer-based replication
OrientDB - OrientDB - The World's First Distributed Multi-Model NoSQL Database with a Graph Database Engine.
FerretDB - FerretDB is an open-source proxy, which is an alternative to MongoDB.
Titan Database - Titan : Distributed Graph Database