Given that JDO's charter is to be datastore-agnostic, a JDO object store should be high on the list. It supports both relational and non-relational databases, including object databases, NoSQL data stores and *literally* anything else.
By leveraging the plugins of DataNucleus, the Apache-licensed open source JDO reference implementation, the following object stores below can be supported out of the box.
RDBMS (all major vendors)
See http://www.datanucleus.org/plugins/ for more info.
Google's BigTable could also be supported thanks to their Google AppEngine for Java persistence solution, which is a DataNucleus plugin, enabling Isis- & JDO-based apps to run on GAEJ.
Note that DN also supports Joda & javax.time datetime types as well.