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
Generic Object Oriented Database System
Distributed thick-client OODBMS using aspect-oriented approach to build transparent interface to database from OO programming languages (C++, Java, C#)
Object-oriented simulation-ready database simultaneously supports Java and C# APIs. Fully-transactional. Uses time-stamps and user ID to track all changes. Supports objects with properties whose values change in time under complex patterns.