CouchDB!

MySQL Community Edition is a freely downloadable version of the world's most popular open source database that is supported by an active community of open source developers and enthusiasts.

MariaDB is a backward compatible, drop-in replacement of the MySQL® Database Server. It includes all major open source storage engines.

MongoDB (from "humongous") is a scalable, high-performance, open source NoSQL database. The database is document-oriented so it manages collections of JSON-like documents. Many applications can thus model data in a more natural way, as data can be nested in complex hierarchies and still be query-able and indexable.

RethinkDB pushes JSON to your apps in realtime.

Apache Cassandra has become the leading NoSQL platform driving many of today's modern business applications by offering continuous availability, high scalability and performance, strong security, and operational simplicity while lowering overall cost of ownership.

Cockroach is a distributed key/value datastore which supports ACID transactional semantics and versioned values as first-class features. The primary design goal is global consistency and survivability, hence the name. Cockroach aims to tolerate disk, machine, rack, and even datacenter failures with minimal latency disruption and no manual intervention. Cockroach nodes are symmetric; a design goal is one binary with minimal configuration and no required auxiliary services.

Couchbase is the NoSQL database market share leader, with production deployments at AOL, Deutsche Post, NTT Docomo, Salesforce.com, Turner Broadcasting Systems, Zynga and hundreds of other organizations worldwide. Couchbase products enable a more flexible, scalable, high-performance, and cost-effective approach to data management than relational database solutions and are particularly well suited for web applications deployed on virtualized or cloud infrastructures, and mobile applications requiring real-time data synchronization between devices and the cloud. Couchbase is a privately held company funded by Accel Partners, Ignition Partners, Mayfield Fund and North Bridge Venture Partners. www.couchbase.com »

ArangoDB database is an open-source NoSQL solution with a flexible data model for documents, graphs, and key-values.

HBase is an open source, non-relational, distributed database modeled after Google's BigTable and is written in Java. It is developed as part of Apache Software Foundation's Apache Hadoop project and runs on top of HDFS (Hadoop Distributed Filesystem), providing BigTable-like capabilities for Hadoop. That is, it provides a fault-tolerant way of storing large quantities of sparse data.

H2 is a relational database management system written in Java. It can be embedded in Java applications or run in the client-server mode. The disk footprint (size of the jar file) is about 1 MB.

The Document-Graph database, SQL and Native Queries, asynchronous commands, intents, and much more.