COMPUTERS : 3D Interfaces again

Greg has pointed out the croquet project. This is a 3D interface, but more from the point of view of navigating through a networked world rather than an efficient place to do work (although the collaboration work was interesting).
In fact this is stressed by the fact that you can expand an entity in the 3D space to fill your entire screen, thereby turning it into a 2D space.
The main examples they give are all operated on a plane that you surf across - like being _in_ the 2D space rather than looking down on it.