Object Databases

Here is what I think about object databases available for Java today.
Those databases are relatively new, but right now they are rapidly gaining grounds. There are two popular object databases available for Java: db4o and Cache.
Cache is a complex and expensive enterprise level solution. I personally like db4o much better. db4o also has GPL license in case you are considering creating open source software and releasing it under GPL also.

db4o is simple to use. They have introduced native queries support not so long ago, which is when you can describe a query with native semantics of the programming language you are using through object oriented API. The big difference with Cache is that they are using SQL to query the data.

I would say db4o went all the way and created object oriented API for their object oriented database. I believe it's more consistent and potentially better performing or may be it's already better performing, I don't know.

Now db4o has database replication system, which does bidirectional synchronization between different instances of db4o database and relational databases. If they did it right, I am sure it will be a great step towards acceptance of this object database in places where only relational database could be.