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#)
The Technology Explorer for IBM DB2 is a light weight, web based console for DB2 that helps you learn about DB2 using demonstrations and tutorials. The TE is designed from the ground up to be easy to use and customize to suit your needs.