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
ERMaster is Eclipse plug-in to make ER diagram. Functionally, Import from data base, Making of table definition book on Excel, Image output, DDL output, etc. are being offered.
JSpeed -- Java-based Open-Source EmbeddedDatabase Management System supporting network database model with limited relational access. JSpeed's goal is to provide a high-speed embeddeddatabase written in 100 percent Java. Key management uses EP-Tree.