Every smartphone today comes with at least one camera. While not rivalling the performance or image quality of a larger DSLR, mobile cameras do perform quite well considering their size limitations. Megapixels aren't the only thing you should look at as it has been proven that MP isn't everything. The size of the sensor (larger is better), the manufacturer of the sensor(Sony's Exmor sensors are the industry leaders) and the aperture of the lens (larger is better) must be taken into consideration. Things like OIS (Optical Image Stabilisation) and two tone flash help in getting smoother videos and proper flash photos.
NFC allows transfer of data between two NFC devices by just touching them. This can be used to transfer photos from a camera and a phone or to make payments via apps and a NFC supported paydesk.
These are found in the high-tier phones which can be used to control various IR devices like projectors and Smart TVs remotely.
Found only in flagship phones, this sensor can be used for unlocking phones with one touch and for authenticating payments.
Some smartphones come with a heart rate sensor at the back which can be used to monitor heart rate.
Just like the heart rate sensor, SpO2 sensor is used to measure the Oxygen levels in blood.