OODBMS - know anything that’s free?

Does anyone know any decent ODBMSes that are free and relatively decent?

I've got some problems to solve (unrelated to the mySQL question) that might be easier in an OODBMS. I've looked a bit, and can't seem to find anything to my liking. Since I already know RDBMSes pretty well, I'm a bit reluctant to go full tilt into some new thing with a huge learning curve. If I can get some kind of "hand-holder" system like MS SQL Server (it's bloody simple) then I'd be happy.

The problem relates to self-joining tables and secondary join tables for hierarchial systems and hierarchial systems where "grand-parents" can be subordinate to "grand-children"... Pretty esoteric for most people...

The real problem has to do with library categorization and existing systems. The Dewey Decimal system and the Library of Congress system are arbitrary, totally linear, and utterly inflexible.

i.e. Items (books) must be uniquely placed within the categorization system, which is completely insane because any given item can be placed in more than one "category".

Am I making any sense to anyone here?