This article on ABC News mentions modifying fluorescent lights to lead brain injury patients to their appointments. The fluorescent lights are modified to flicker at a high unnoticable rate to transmit binary coordinate data via light signals to a handheld computer. The handheld computer receives the light signal data, determines the current location and can then direct a brain injury patient where to go. The directions are given by a simple "you are located here you need to go there" sort of method. The computer can even speak the directions to the patient. Using this system, brain injury patients get to their appointments more often. It's interesting, of course, because such a system could also be used to help robots find their way in a building. This Article from MIT's TechnololgyReview.com goes into more detail of how the invention works. Other articles on the subject here, here.