A lot has been written about employers giving their workers cell phones with GPS-enabled geo-locator services that lets them keep track of the whereabouts of the workers at all times through the cell phone. These services work with phones with GPS integrated into the device and track the workers by transmitting the exact location of the worker at certain intervals.
An enterprising German inventor, Aram Bartholl, has developed a couple of pouches that create Faraday cages that block the transmission of the location data back to the service. Theoretically this could prevent the employer from knowing where you are, which would probably get you fired. Interesting use of technology, though.