This 3D display uses a single bead controlled by acoustic levitation

Researchers at Sussex University have developed a 3D model with a single "pixel" -- a tiny white plastic bead that's quickly moved around the space inside a black box by an array of ultrasound speakers. A colored light source projects light onto the fast moving bead. The end result is that the bead draws "shapes in the air."

[via Core77]

Image: YouTube