Last summer semester I created a graph visualization application for a university lecture. I have used CeGUI for the user interface. You can get more information, as well as the GPL sourcecode here:
http://www.cg.tuwien.ac.at/courses/Info ... Embedders/
The main feature is force-based layout of graphs using a spring embedder algorithm, utilizing non-euclidian spaces (hyperbolic and spherical).
I would be glad to answer any questions
Oh, and sorry, the users guide is only available in german, but you will probably figure out how to use it anyway.