||Object-Relational Mapping with Apache Jakarta OJB|
<quote>Unfortunately, there is no complete, free implementation of JDO today</quote>
I don't think that's correct: TJDO has an Apache license, so it's free. Some collections are not implemented yet but it seems to me a production-ready JDO implementation for a lot of uses.
They are in the process of extending TJDO to more databases than just Oracle and making sure it can be plugged into an applicationserver.
Go check it out: http://www.sourceforge.net/projects/tjdo
Btw: I'm not associated to TJDO. Just enthousiastic about JDO/TJDO.