There is Firebird which is essentially an opensouce InterBase (IIRC it is forked from IB5 source). It is probably more widely used than new commercial versions

There is Firebird which is essentially an opensouce InterBase (IIRC it is forked from IB5 source). It is probably more widely used than new commercial versions of IB. One interesting feature is that it is probably only opensource DBMS that not only works with multiple engines accessing same database stored on Windows share but it is actually supported usecase.