With Eclipse GEF 3.4 M4 comes a new component: Zest.
Zest is a visualization toolkit for Eclipse. The primary goal of Zest is to make graph based programming easy. Using Zest, Graphs are considered SWT Components which have been wrapped using standard JFace viewers. This allows developers to use Zest the same way they use JFace Tables, Trees and Lists.
The Zest Wiki has a simple example showing what you can do.
The best full example of zest is the PDE Incubator Dependency Visualization . The dependency visualization provides a view to help you see the dependency graph for your favorite plug-in.