The FitBit is designed to be worn all day and night on a belt loop, bra strap, or free pocket. It boasts that it can track your caloric expediture, has a Wiimotesque motion sensor to keep track steps taken and distance traveled, wear on your wrist at night to monitor how well you're sleeping, and has 10 days of battery life on a full charge. All this from a device no bigger than a thumb drive.
After it gathers all of this info it automatically loads it to a website (after you walk within range of it base station) where you can keep track of your stats. So you can see if that WiiFit exercise is really doing anything for you.
It's not set to release until around Christmas time and then it'll be $99.

Would you like to know more?