Possibly the most exciting new NetBeans plugin that's under development is the JPA Modeler, led by Gaurav Gupta, who also works on the Business Process Modeling plugin for NetBeans:
Watch this short movie to see it in action. In its latest incarnation, available from the link above, you can reverse engineer a diagram from existing JPA sources, which is also amazing, and let's you use the JPA modeler on old sources, i.e., no need to create a new project from scratch to use this plugin. And, whenever the diagram becomes out of date, you can simply regenerate it, from the current state of your sources. I've tried it and it delivers exactly what I had expected. Plus, it looks seriously professional and very polished, as this annotated YouTube clip shows:
If you watch the above, you'll see right near the end that you can change the diagram, then regenerate the tables in your database, and recreate the JPA classes. I.e., this lets you visually model your data and then generate all the database content and Java classes from the diagram. Really powerful.
Also, follow the JPA Modeler YouTube Channel, there's lots of content there, constantly being updated with new movies.