The iriverON Heart Rate Monitoring Bluetooth Headset makes a unique claim for a heart rate monitor, at least that I've seen: it can measure V02 Max.

V02 Max is a measurement of the maximum oxygen your body can deliver to its muscles and consume during exertion. Usually, to test this figure, you have to go a lab where they put a mask over your face and have you run on a treadmill. Aerobic exercise can improve your V02 Max.

Endurance athletes bandy V02 Max figures about like bodybuilders discuss body fat percentages or basketball players discuss vertical leaps. The average person has a V02 Max in the 30 to 40 range (ml/kg/min). As a point of comparison, some of the great aerobic athletes of all time have had tested V02 Max of 80 and higher. Miguel Indurain, the great cyclist, was said to have confirmed V02 Max of 88.

There is more to being world class at a sport than just a high V02 Max, one's lactate threshold matters, too, but to date only professional or very serious athletes have had access to regular V02 Max testing during training. I've always been curious to see where I stack up and how my figure shifts from working out, but traveling to a testing facility and paying hundreds of dollars for each test has always been prohibitive.

I have no idea how accurate the iriverON measure of V02 Max is, but I'm going to give it a try and will report back here. The methodology is one I've never heard of; CEO Steven LeBoeuf (no relation to Shia, I hope), explains in this post:

Valencell has designed a highly miniaturized sensor module that is capable of fitting inside virtually any earbud or audio headset. The sensor module shines light into the ear region and measures how this light interacts with blood flow.
This information is then processed by novel signal extraction algorithms to pull out blood flow information (which is actually very faint) from what amounts to be an incredible amount of noise. For example, the signal from blood flow is more than 100-1,000 times weaker than the signal coming from motion noise and environmental noise (like sunlight).
Next, Valencell’s novel algorithms process this information into important vital signs metrics, such as heart rate, respiration rate, energy expenditure, and more. These vital signs metrics are then sent wirelessly to select smartphone applications (Android and iOS devices) that generate real-time fitness assessments such as resting heart rate, training load, VO2max, personalized heart rate zones, and recovery time.

I have yet to hear of a more convenient way to measure one's lactate threshold that doesn't involve running on a treadmill and continually pricking your fingertip to draw blood.