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
DOMSafeXML is an XML database platform based on MonetDB. It offers multi-user access to XML files via DOM interface. Language bindings for COM, C++, Java and C# are available.