Sean Gerety come with a strange but cool idea of building a multi-direction treeview. I am not sure its really practical and user friendly but it's something nice ;-)
Also about navigation I remember also about the very nice navigation idea that a company has in a Tablet PC killer application (real words from Julia !).
I hope some day I will have sometime to think about doing that as .Net project, sounds really fab. It's kid of a floating pie chart, each sector representing a clickable selection, link to a sub level, and to the menu execution.
Well check by yourself the video, if you think I am speaking gibberish.