From Joystiq:

Why doesn’t Wii Fit include an estimate of the calories you burn during exercises? That would be really useful. Nintendo seems to agree, at least — it has created a Flash application that does just that.

The Wii Fit Calorie Meter couldn’t be easier to use, unless it were in English! Push the big green button in the middle to start, then enter your weight (in kilograms). You can then click on each exercise you’ve done (or plan to do) and tell the calculator the number of sets you’ve done of each. The app will add up your calories as you go!