There are several sensors embedded in smart phones, and that number is set to increase as we come to depend more on them.GPS sensors are used for tracking and thus indispensable for applications like Waze, which is used to plot travel routes and which gives indications of the traffic conditions. Magnetic sensors are used for "Compass Apps" and the list goes on.
One area that is getting a fair amount of attention is that of health care. Fitted with the sensors, add-ons and software, they can become cost-effective, sophisticated and portable medical equipment. This would facilitate health care, not only in developing countries but also in the rural areas of developed ones, as they can transmit data to hospitals locally and internationally.
In 2009, the MIT Technology review reported on a phone-based device called Cellscope. In essence this was a portable microscope. Using a physical attachment to the phone, it was possible to perform simple analyses on body fluids, the results of which were transmitted for storage and further analysis elsewhere. The intent was to identify and track diseases like TB and malaria. This, in fact, was a class project at the University of California, Berkeley.
http://www.guardian.co.tt/digital/new-members