Piccolo2D developers, I've built a visual tracing tool for Java. It connects to a running JVM and generates an interactive sequence diagram by tracing the running code. The visualization is done using Piccolo2D. If you are a Piccolo2D developer and would like a free license, let me know.
Best, Aaron -- Piccolo2D Developers Group: http://groups.google.com/group/piccolo2d-dev?hl=en