Although still in the development stage, this project by Instructables member DangerousTim is an interesting addition to the world of Arduino. It was developed as the cost of medical equipment in rural Africa and Asia was prohibitive - and without access to a reliable power supply this will offer the end user a more inexpensive way of allowing people to track their heart rate.
The monitor uses the principle of photoplethysmography (PPG), which "... is a non-invasive method of measuring the variation in blood volume in tissues using a light source and a detector. Since the change in blood volume is synchronous to the heart beat, this technique can be used to calculate the heart rate". A quick demonstration is shown in the following video:
This is a great, non-invasive solution to a very common problem - so visit the Instructable page to follow the project. And for more, we're on facebook, Google+, and twitter - so follow us for news and product updates as well.