What are the possible uses of sensor of mobile phone? Not just common uses, but more extreme examples?
Although,I can't get what do you mean by extreme examples,but you can make use out of the phone sensors in some possible scenarios as -
Consider a Galaxy S4 phone,which has wide array of sensors.
1) The gesture sensor can be used to change the presentation slide when the phone is connected to a laptop via Wi-Fi.
2) The light sensor can be used to automatically adjust phone's brightness so that the user's eyes doesn't gets too much strain in any lighting environment.
3) The barometer sensor senses environment pressure.It can be useful for asthmatic people or any people who may find breathing problems in any adverse environment.It's accompanied via S health app.
4) temperature/Humidity sensor allows on staying in different environment to take necessary precautions before getting physically ill and suffer from dehydration or hypothermia.
5) The geomagnetic sensor can be useful for those who want to be sure that surrounding environment's magnetic field is not high enough to interfere with the pacemaker or any other artificial medical equipment that is implanted into its body.
It can also be used to study affects of magnetic field in nearby devices.