As the Arduino and compatible boards have several easily-accessible analogue input pins, you can use sensors to measure all manners of data and work with the results. One example of this is to make a pulse-sensor described by Angelo from ascas.com. His version builds on a previous tutorial, and shows how to make your own clip sensor by modifying the clamp from a certain type of coathanger.
Furthermore in case the signal strength is too weak, an amplifier circuit is provided. The sensor data can the be sent to a PC via USB and displayed using processing for a neat display as shown below:
Although you shouldn't rely on home-made medical sensors for medical diagnosis (see a Doctor instead) it's a fun project, so for complete details visit Angelo's Instructable. And for more, we're on facebook, Google+, and twitter - so follow us for news and product updates as well.
If you're looking for an Arduino Uno-compatible board for various projects, choose what tens of thousands of others have done and use our Freetronics Eleven - the Arduino-Uno compatible with low-profile USB socket, onboard prototyping space and easy to view LEDs: