The Cardiio app is an iPhone application that can measure one's heart rate. The application uses technology developed by MIT that is capable of measuring heart rate by using the camera on the iPhone to track changes in how one's face reflects light, which leads to an accurate heart rate measurement. With this technology, no sensors are needed.
The Cardiio app also comes with some cool other features. One can track one's heart rate history to see changes over time. The app also can estimate one's life expectancy, and compare such statistics to other peoples' or animals' -- all this comes with a beautifully designed interface!
While the technology is fairly accurate, the makers of the app don't intend Cardiio to replace seeing the doctor or taking other heart rate measurements. It is a fun app to supplement these things though, or use for fitness.