It's not quite X-ray vision, but this portable radar can penetrate walls to detect people moving on the other side.
A device developed by U.K. technology company Cambridge Consultants provides a 3D image that indicates activity more than 65 feet away, according to an article in New Scientist Tech. (Cambridge provides videos of the "Prism 200" being used for spot checks and for longer surveillance.)
The radar isn't foolproof: Metal beams can present a problem, and a person who remains perfectly still might not show up, though movement from breathing can sometimes be detected. Other technologies can provide more sophisticated information, of course, but the key to the "Prism 200" is its size--about as big as a backpack and weighing only 11 pounds. That means it could be a life-saving tool for soldiers, police officers, rescue workers and even the occasional dad whose children are always trying to scare him into a heart attack.