If you're looking to explore Java's most inner workings, the OpenJDK is surely suited for such a purpose, since it provides a complete open source implementation of the JDK. This blog entry by David Herron talks about the project and its latest release.
This is a very early release, there are bugs, and if you build from these sources they do not yet produce a compliant binary. The latter is being worked on.
Another recent related and important bit of news is the clearing of a major encumbrance .. [OpenJDK 2D-Dev] imaging and color classes binary plugs no longer needed (soon) discusses how we've recently gotten the rights necessary to relicense under the GPL the source for some classes critical to image and graphics rendering. The code is not yet in the OpenJDK6 release, we expect to get it there soon.
This 'end' is just the beginning of a new phase, because there is more work ahead in this. We have made an important step, but we are on a journey and this is only one of the steps along this path.
Read David Herron's post: