Another interesting concept that has been floated before is a Zoomable User Interface (ZUI) and the best / most complete one I've seen is called Eagle Mode. Probably more of a concept piece than anything, but having used it a fair bit, I appreciate the concept a lot and thought it might sit as another idea to consider. Feels a bit exhausting in some regards, but does very well in terms of completely turning the idea of a GUI on its head. Another important note is that this one isn't meant as a replacement, it currently exists as an application that still runs atop an existing GUI. Haven't tried their Windows variant as I live on Linux.
YouTube Demo: https://www.youtube.com/watch?v=G6yPQKt3mBA
Source for Project: http://eaglemode.sourceforge.net/index.html
Screenshots:
