Another feature that Park believes sets Fitbit apart is the way that its Web service automatically converts steps taken into burned calories and lets people compare their activity with that of other Fitbit users. "I can get a real-time activity feed from my friends," says Park. The site can also log meals and create calorie budgets to tie into a diet regime.

At night, the sensor fits into a wristband, and its accelerometer tracks tiny tremors in the wrist that correlate to different stages of sleep. When sleep-related data is uploaded to the Web, it is used to create a graph showing the amount and quality of sleep achieved each night.

Park says that Fitbit isn't meant to replace a sports pedometer; rather, it's meant to give people a better sense of their daily activity and act as a dieting aid. The sensor will be available by December or January, Park says, and will retail for $99; use of the Fitbit website will be free.