HierarchicalDataBases are a form of DataBase predating RelationalDataBases and PostRelationalDataBases but still used in places where applications have and need for an indexed database of relatively complex data. Recently HierarchicalDataBases have been loosing ground/marketshare to XMLDataBases.
The two OpenSource implmentations are BerkeleyDB and GDBM, which both implement the UNIX dbm interface and can be used (almost) interchangably. The Windows registry is also a HierarchicalDataBase.
LDAP is a HierarchicalDataBase access standard, in much the same way that SQL is a RelationalDataBase access standard.