Jgrasp has a really cool debug viewer: http://www.jgrasp.org/viewers.html
Without any setup, you can see how your objects are interacting visually. It's been really handy for working with linked lists.
However, overall, I hate jGrasp. Is there anything like this for Netbeans or Eclipse? I ran a number of searches and came up dry. It's too bad....
I think, the closest thing you'll get is Jive : http://www.cse.buffalo.edu/jive/
It's a plugin for Eclipse. See there for installing instructions : http://www.cse.buffalo.edu/jive/download.html
In case anyone ends up here looking for an answer, jGRASP viewer/canvas plugins for Eclipse and IntelliJ have been available for a few years.
