Firebird RDBMS offers ANSI SQL features & runs on Linux, Windows & several Unix platforms. Features excellent concurrency & performance & powerful stored procedure & trigger language.
More information and downloads at https://firebirdsql.org/, issue tracker is https://github.com/FirebirdSQL/firebird/issues
EyeDB is an Object Oriented Database Management System (OODBMS) providing an advanced object model (inheritance, collections, arrays, methods, triggers, constraints), an object definition language, an object query language and C++ and Java bindings.
JMigre is a 'universal' databases convertor based on Java and JDBC.
It should be able to convert any database from any database management system (DBMS) supporting JDBC, to another DBMS supporting JDBC (on any OperatingSystem supporting Java).