Fitbit's Alta HR packs a heart rate monitor, but is small enough to forget you have it on - SoyaCincau
Fitbit Alta HR: The Slimmest Heart Rate Monitor Yet - Techlicious
senQ Digital Station - Monitor your health more effectively with Fitbit Charge HR (Large) FIT-FB405BKL. Available for online purchase at http://goo.gl/UXRnPc | Facebook