Source hereIf you've ever hooked up a device to a wifi network you'll be aware of the screen which shows you the available wifi networks in your area, and their names. But with the right software on your PC or laptop, you can pull much more data out of the air than merely a list of passing signals. WAY more information, actually. All you need is some wifi analysis software and, of course, a machine (desktop or laptop) that has wifi hardware built in and enabled.
Acrylic can be downloaded here and it's a 6 MB download.
The program is malware-free according to VirusTotal and Web of Trust.