New ways to present heirarchical (and bi-directional heirarchical) data on the web is a passion of mine. I still think there are better ways than breadcrumbs (yahoo), hyperbolic links (inxight, and HotSauce MCF), and heirarchical menus (everywhere). One interesting presentation can be found at Links 2 Go. They give you visibility up and/or down two heirarchical levels depending on how populated those levels are (GUI example).

This system works well for me, but depends on a relatively small and relatively constant branching factor, otherwise there are simply too many links going into and out of each node. With a well-balanced tree, you don't need to worry about other list-navigation systems like fisheye lenses (UMD: HCIL, MacOS X Dock).

A combination of a balanced heirarchy, directed entry point into the heirarchy (like when Yahoo's search returns categories before sites) and a lens-based analog navigation might be just the thing for a future OS. We could finally put our video cards to good use!