The Galaxy S5 claimed to be the first phone with a heart rate sensor when it was announced. However, this may be only true in terms of hardware, since every phone with a LED flash and a camera can measure your heart rate, too.
The LED lights through your finger and the camera detects you heart rate – simple as that. In the Windows Phone store there are apps for this too. One of them is Instant Heart Rate.
There are only a few things to say about it: It works, it works quickly, and it looks great. That is everything. There is a free trial available and if you want to buy it, you have to pay 1.99$, which is quite much for such an app, but not totally unfair.
Otherwise, if you want a free app, which works the same, only is a little slower and doesn’t look that good, you can check out Heart Rate.
Not so innovative anymore, what, Samesung? 😉