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#)
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.
picoSQL is an Italian multi-user relational database management system with transactions support. Because it derives from a commercial project, it is already robust, fast, and rather complete.