Meet the MetroFlash Echo: a Bluetooth heart rate sensor that combines with dozens of popular fitness apps to provide you with accurate heart rate data. It is compatible with iOS and Android devices and offers up to 10 hours of battery life. You will have to wear it on your wrist to get started.
Echo is waterproof to 10 meters and has a range of up to 10 meters. It supports popular apps such as RunKeeper, MapMyRun, Endomondo, and Wahoo Fitness. Costs under $50.