Here’s a great deal on this Polar H7 Bluetooth Heart Rate Sensor and Fitness Tracker.  Today only (and while supplies last), you can get one for just $34.95.

• HEART RATE MONITOR: Provides live, accurate heart rate to compatible mobile training apps through Bluetooth Smart technology
• FITNESS TRACKER: Waterproof heart rate sensor, Compatible with iPhone 4S, iPhone 5, iPod touch (5th generation), iPad (3rd and 4th generation), iPad mini and iPod Nano (7th generation)Bluetooth Smart transmission technology
• STRONG BATTERY LIFE: For optimal battery life, detach transmitter from chest strap after every use
• ADVANCED TECHNOLOGY: Gym Link connects Polar heart rate monitors and activity trackers with compatible gym equipment
• CALORIE COUNTER CAPABILITIES: For those who want to count the calories they burn and get basic heart rate-based features to keep their fitness training simple

I recently purchased one of these (for more than this sadly) to test my heart rate variability each morning and track my exercise recovery and stress.  You can learn more about HRV here.  I’m also using it to build an aerobic base using the MAF method that I learned about in the book Primal Endurance.