I have just purchased a FitBit One, and I must say I love it. Basically it’s a small unobtrusive device that you carry around with you, and it measures your activity and gives you feed back, via the Fitbit website, on your progress.
It counts the number of steps you do during your day and the number of floors climbed, this is converted into a calorific figure which can then be added to your BMR (Basal Metabolic Rate), to give you a net calories burned for the day. If you link this website with your MyFitnessPal website it will give you a net calories remaining figure for that day. This should allow you to control you calorie intake, no matter what your activity has been like that day.
Another function of the device is to monitor your sleep pattern. A good way to see how dieting is affecting your sleep.