Fitbit Flex
Credit: Photograph by Plamen Petkov

A narrow sensor fitted into a removable rubber bracelet, the Fitbit Flex is light and inconspicuous, and accurately tracks steps, calories burned, and sleep (it did overcount on certain days). It could be seen as a better version of Nike's FuelBand, which tracks only steps and calories. We wish it had an LCD screen and an altimeter (it doesn't track stairs traveled). [$100;]