
Erich Gamma and Kent Beck have written a graphical object explorer plugin for Eclipse, which is available at the Javaspider site.
Above is an example of how you can interactively inspect the structure of a running application, in this case, Eclipse itself and it's plugin registry.
This is really useful when browsing/debugging large and complex data structures, and it reminds me of a similar feature in DDD that I use to use a lot at University when debugging C/C++ applications.