The prefuse visualization toolkit is Jeffrey Heer's hugely impressive collection of graph layout tools for use in Java applications/applets. I recommend watching the demo (30MB DivX AVI), and perhaps reading the paper (PDF) which provides a more detailed description. The files themselves are still at the alpha stage, but are available from SourceForge.
SpaceTree was impressive, yesterday, but the expanding, zooming hierarchical browser is just one of the layouts that prefuse can handle.