Open source object store database for Java/Ruby/Python (10gen.com)

It isn't ready for prime-time yet, but unlike other object databases, the licensing is clear (though not ideal IMHO--AGPL3) and it's under active development.

Here's the Github repo: http://github.com/10gen/mongo/tree/master

And the architecture: http://www.10gen.com/wiki/db.guide.02_architecture