Using a mirror spinning at a ridiculous 20Hz—20 revolutions a second—researchers at the USC Institute for Creative Technologies have created a fully-functioning monochrome “holographic” display. While holographic displays are pretty common, most implementations don’t allow true 360-degree walkarounds. This puppy does.
I was going to pooh-pooh the practicality of home implementations, but surrounding the spinning mirror with a protective bubble wouldn’t be that impractical. Getting the overhead projector with beams in the image on the mirror into something more compact would likely be a bigger challenge.
However you slice it: very, very cool.