Db4objects
Db4o is an open source object database engine - a tool for software developers, designed to be as simple and fast as possible. it is supplied as a programming library (java: .jar /.net: .dll) to be included with developer applications.
http://www.db4o.com/
Berkeley DB
Berkeley DB
http://www.sleepycat.com/
Firebird
Firebird is a relational database offering many ansi sql-92 features that runs on linux, windows, and a variety of unix platforms. firebird offers excellent concurrency, high performance, and powerful language support for stored procedures and triggers. it has been used in production systems, under a variety of names since 1981. firebird is a commercially independent project of c and c++ programmers, technical advisors and supporters developing and enhancing a multi-platform relational database management system based on the source code released by inprise corp (now known as borland software corp) under the interbase public license v.1.0 on 25 july, 2000.
http://www.firebirdsql.org/
GNU SQL
Gnu sql server home page
http://www.ispras.ru/~gsql/
MySQL
MySQL
http://www.mysql.com/
MSQL (miniSQL)
mSQL (miniSQL)
http://www.hughes.com.au/
Postgres
Postgres
http://www.postgresql.org/