Are there opensource hierarchical database management systems
Since XML databases are hierarchical, you could look at those. I recently used BaseX and liked it a lot.
Any LDAP implementation is going to be hierarchical, or at least exposed as such, even if the backend's something else.
- OpenLDAP.
- Apache Directory
(There's likely others, and even Apple's Open Directory, but I don't know if that one has a dedicated project page)
I found mumps and its GPL licensed.*Mumps (also referred to as M) is a general purpose programming language that supports a unique, hierarchical (or multidimensional) database facility * http://cs.uni.edu/~okane/mumps.html
Update
Important Links.
-http://gradvs1.mgateway.com/main/
-http://www.intersystems.com/