Scientists from the University of Tokyo have created what they're calling an Airborne Ultrasound Tactile Display, that's basically a touchable holographic 3D display. Integrating a concave mirror onto which an LCD projector creates the image part of the system, together with an acoustic radiation pressure generator, the display can generate the feel of coming into physical contact with projected 3D objects as the user moves their hand around.
Video demo after the cut
The technical explanation of the ultrasound system is, frankly, too complex for these pages, but a paper describing the setup in more detail is available here [pdf link]. On a basic level, adjusting the ultrasound tranducer array built into the concave mirror assembly allows the system to create different pressure fields; since the acoustic impedance of the air is so different to that of a user's hand, 99.9-percent of the incident acoustic energy is impacted onto the hand.
In short, you can feel sensations such as water dripping, textured surfaces and other tactile feedback where there is no actual object, and the ultrasound system does not impede or affect the holographic display. No word on what the practical applications to this may be, but one possibility could be in-vehicle controls that respond to gesture rather than a physical button or touchscreen, and which use ultrasonics to create invisible controls you do not need to take your eyes off the road to use.