I'm not sure if this has been done before but I'd made a graphical map and an expandable map of the packages in Java:
Java Package Graphical Map
I had used Freemind, my favourite concept-mapping software, to this. It was just an idea of mine that i had came up with one day running through some documentation. It's not completed but i wanted to get some feedback before i do anything else with it.
My ideas for the next version:
+Color the elements using some common color scheme: green for classes, purple for interfaces, and some kinda gold/tan kinda color for enums. Iv'e noticed that Freemind has some 'physical states', which apply formatting, match up with these colors. 'OK' for classes, 'Bad' for interfaces, and 'Question' for enums.
+Also making an inheritance diagram of classes and enums, also with the same colors
Edit: The links are now broken, as the documentation is not yet static and will be changing often.
Edit2: If i do continute with this, I plan on mapping apache commons instead.