LJV.java is tool for visualizing Java data structures, using Graphviz. It has been used to good effect in a CS1 and CS2-level data structures course. The software is available under the GNU General Public License. Documentation for the system is in preparation. Meanwhile, there are some examples, and a paper I wrote on the system for the ACE 2004 conference (slides). If you find this tool helpful, please send me an email.
Kyle Gillette has put together some material for AP Computer Science teachers that should be useful to anyone wanting a gentle introduction to the tool.
John Hamer