A favorite startup of mine is bodybugg (http://www.bodybugg.com/). They measure the flux of body heat and determine a person's caloric output. I built some MEMS devices once and was juiced about building a body calorimeter. Too bad I had my idea something like three years after bodybugg went to market :)

I didn't know that was possible. Is there a page that says how it works? I don't want to watch their video.

I don't know for sure. My best guess is they have a calorimeter, a device which measures the irradiance on a surface. The Bodybugg companies then does a series of calibrations to convert the irradiance into the power the body outputs. The power used per (second?) is integrated up over the course of a day, and converted into calories.

Essentially, it's a fancy thermometer.

Never heard of such a device. You really do learn something new everyday!