In the context of providing class diagram visualization for TextUML Toolkit, I have developed a simple graphical viewing framework for Eclipse. It is content type based, and allows you to view anything a content provider has been registered for. For instance, any image file supported by SWT:
But you can also view the graphical representation of a Graphviz DOT file, and it will even update as you edit the file:
And finally, and also the reason why I had to develop support for DOT visualization in the first place, you can also visualize a UML model (here showing a model created using the TextUML Toolkit):
All these features (except for the TextUML Toolkit itself) are part of the EclipseGraphviz project, which is open source (EPL). No releases yet as this is still very new, but you can grab the source from the Subversion repository. If you would like to contribute to the EclipseGraphviz project, or to the graphical viewing framework, your help will be most welcome.