H2 Database Engine!

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.

Base is a full-featured desktop database front end, designed to meet the needs of a broad array of users, for all kinds of usages, such as keeping track of collections of objects,maintaining customer information databases,maintaining student grade and curriculum databases,storing survey information and experimentation data,storing population and census data,producing monthly financial reports in companies and organizations, managing audit, quality assurance and production data.

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.

FileMaker Pro is powerful, easy-to-use database software that helps you and your team get any task done faster. Millions of people in business, government, and education use FileMaker Pro to effortlessly manage all their information on Windows, Mac, and the web.

Oracle Database delivers industry leading performance, scalability, security and reliability on a choice of clustered or single-servers running Windows, Linux, and UNIX. It provides comprehensive features to easily manage the most demanding transaction processing, business intelligence, and content management applications.

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 »

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.