First up is the Heliodisplay which is ready to go technology at a cost of $20,000. Heliodisplay is based around patented and patent pending technology to transform normal ambient air and display video images into free-space. The YouTube video embedded below provides a good overview:
Similar to the Heliodisplay is the Fogscreen which uses water droplets as the medium for projection. Fogscreen is literally just a screen and any projector can be used, with this in mind it is mainly aimed at 'walkthrough' advertising - Beyond Tomorrow take a look at the technology below:
Both Heliodisplay and Fogscreen are flat surface technologies, to move towards a full holographic display you need to be able to view the image from any angle. CheoOptics's technology uses a pyramidal structure to enable a 360 degree image to be projected up to 30 meters in size. While not suited to the home environment the movie below provides an intriguing insight into where the technology is moving:
To step back to smaller scale a floating Virtual Globe is desirable, especially if sometime in the future there is a way to project Google Earth type information into the real-space. A basic example can be made using spinning LED's a per the movie below:
Looking down the line at small-form display technology, Hitachi has announced news of a new device utilising an array of mirrors to imagery in a manner that creates the illusion of a three dimensional image - as pictured:
Read more about the stereoscopic display via Hitachi (Japanese link)
Holographic and stereoscopic displays are gradually reaching the marketplace, indeed, it looks like within 5 years we may well be able to view a digital earth or 3D model of a city floating in real-space.
With this technology comes the possibility to visualise and communicate data in new and exciting ways, link this with mutli-user worlds ala Second Life and it suddenly becomes a 3D multi-user network displayed in cartesian space.