Eclipse ZEST is a very cool Framework based on Eclipse GEF, for creating graph based software visualization tools for eclipse, with a few lines of code. The PDE Dependency Visualization Plugin for example is based on ZEST, see also my pervious post here.
Good ZEST starting points are the nice examples apps in the Eclipse ZEST CVS.
Here a screenshot of my first ZEST demo application.
Links:
http://www.eclipse.org/gef/zest/
http://github.com/tux2323/bundleviewer
http://www.eclipse.org/gef/zest/snippets.php