The FitBit is a simple clip-on device with a built-in motion sensor that serves as a information-age pedometer, tracking exxercise intensity levels, sleep quality, calories burned, and distanced travelled — all beamed back wirelessly to the internet (via your Wi-Fi network, I presume) to the FitBit website. There you can compare stats with other FitBit users to see how well you're doing. The whole package will be sold for $100 this Fall.
There may be other incentives besides peer review, explains Gearlog:
The current FitBit Device has no display, but Park promises that the final product, expected this December, will feature an OLED display. The readout will include, among other things, a user-definable avatar that will shrink and grow to indication your progress. Kind of like a Tamagotchi, except you're the one you're keeping alive.
This may end up sounding like nit-picking but it's really not: I wish there were a way for the FitBit to track your weight automatically, too.