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Free Database Servers, Engines, Software

MongoDB
MongoDB is an open-source document database, written in C++, MongoDB uses JSON-style documents with dynamic schemas

CouchDB
CouchDB stores your data with JSON documents. These can be accessed with your web browser, via HTTP. Query, combine, and transform your documents with JavaScript. According to the website, you can even serve web apps directly out of Couch.

Apache Derby
Apache Derby, an Apache DB subproject, is an open source relational database implemented entirely in Java and available under the Apache License, Version 2.0.

ScimoreDB
ScimoreDB Distributed SQL database is free shared-nothing clustered database system .

Perst
Perst is an object-oriented embedded database for applications that need to deal with persistent data. There are two implementations of Perst - pure Java and pure C# (C Sharp).

H2
H2 is a SQL database engine. Available as Java library and as native executable (using GCJ). Clustering, embedded and server mode, disk based or in-memory operation. JDBC and (partially) ODBC APIs are supported. Support for transactions, referential integrity, triggers, subqueries, views, and encryption. A browser based console application is included. H2 stands for 'Hypersonic 2' and is written by original author of Hypersonic SQL.

HSQLDB
HSQLDB is the leading SQL relational database engine written in Java. It has a JDBC driver and supports a rich subset of ANSI-92 SQL (BNF tree format) plus SQL 99 and 2003 enhancements. It offers a small, fast database engine which offers both in-memory and disk-based tables and supports embedded and server modes. Additionally, it includes tools such as a minimal web server, in-memory query and management tools and a number of demonstration examples.

db4o
db4o (database for objects) is the open source object database for Java and .NET. It is free software available under the GPL and also under a commercial license. It can persist any native object with a single line of code and allows you to experience true object-orientation. Features include: ACID transactions, query-by-example, S.O.D.A. query API and automatic class schema recognition, small footprint.

Ingres
Ingres is a proven, high-performance open source relational database solution that offers the scalability, integration and flexibility to leverage information resources across a wide range of platforms, including Linux, UNIX, Windows and OpenVMS.

SQLite
SQLite is a small C library that implements a self-contained, embeddable, zero-configuration SQL database engine

Firebird
Firebird is an OpenSource database environment formerly known as Borland Interbase Version 6. Features include full SQL compliancy, stored procedures, and other advance database features.

C#-SQLite
Windows C# port of the SQLite library released under an MIT license.

MySQL database server and standard clients
MySQL is available for many platforms and you can download the server and client software as well as a MySQL ODBC diver and a bunch of other MySQL tools from this page.

postgresql
PostgreSQL is an open source relational database management system and runs on many platforms. You can download the latest release as well as a conversion utility (MySQL to PostgreSQL) from this page.

Hypersonic SQL
hsqldb is a relational database engine written in Java, with a JDBC driver, supporting a rich subset of ANSI-92 SQL (BNF tree format). It offers a small (less than 160k), fast database engine which offers both in memory and disk based tables. Embedded and server modes are available. Additionally, it includes tools such as a minimal web server, in-memory query and management tools (can be run as applets) and a number of demonstration examples. (100% Java Database)

GNU SQL Server
SQL Server is a free portable multiuser relatational database management system. It supports the full SQL89 dialect and has some extensions from SQL92. It provides multiuser access and transaction isolation based on predicative locks. The working OS: UNIX. The working language: C . It also uses RPC, shared memory and message queues.