If you have ever wanted to see through walls, but couldn't afford the expensive equipment to do so, then some MIT researchers deserve your attention. They've devised a system called "Wi-Vi", which is designed similarly to sonar where a signal is blasted out and bounced back. An image is then created based on what it hits, and if it happens to be a moving object, then that will become visible on the display immediately.
At the moment, the technology is in its infancy, and so not surprisingly, the resolution of the data is small... very small. You'll clearly be able to see movement in another room, but as for distinguishing whether or not it's a male, female, or perhaps even a big dog, might be a little hard to do. The researchers are not too concerned with this, though, as it's movement in general that's important to monitor. Think search and rescue. Another example is given of an elderly person - this would give an easy way to check-up quick and make sure they are still mobile.
The unfortunate thing about Wi-Vi is that it requires two separate devices - one to send a regular signal, the other to send an inverse. With the researchers' technology in place, these signals would work in unison to build an image similar to the one you see above. If someone wanted to use Wi-Vi for spying into a room (which again would do little more at the moment than let you know someone's there), they'd need to put one Wi-Vi device on one side of the wall, and then another on the other.
Like Google Glass, Wi-Vi could be used for both good and bad. Imagine potential thieves, for example, scouting out a house. With two people - one on either side of the house - it could very well be easy to scan inside and see if there is any activity or not. A bit scary when you think about it. On the flipside, technology like this does have huge potential... it's effectively radar technology that anyone can afford.
To make use of it, though, current Wi-Fi modules cannot be used. Instead, special Wi-Vi ones would be, which means our smartphone vendors would have to want to install them. We'll see if that turns out to be the case.