CouchBase!

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.

Apache CouchDB is a distributed, fault-tolerant and schema-free document-oriented database accessible via a RESTful HTTP / JSON API. It includes incremental map / reduce mechanics for queries and keeps data safe with an append-only datastore.

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.

Orchestrate Real Apps on the Cloud with CloudifyAchieve a smooth transition to the cloud and easy automation of even the most complex applications throughout their entire lifecycle with Cloudify. Orchestrate the creation of the whole cloud infrastructure required for your application, starting from compute resources all the way down to networks and block storage devices. Cloudify will then deploy your applications to the cloud (OpenStack, VMWare vSphere, or even bare metal like Softlayer), monitor their progress and scale them when needed. »

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

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.

Free & open source, high-performance, distributed memory object caching system, generic in nature, but intended for use in speeding up dynamic web applications by alleviating database load.

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.