I'm a big fan of my Fitbit pedometer because it does most of its work without any interaction. I clip it onto my pocket and it counts my steps and flights of stairs as I walk throughout the day, then automatically, wirelessly uploads the data to Fitbit's website whenever I'm within range of its USB dongle plugged into one of my computers. The whole thing works without having to think about it or plug anything in. The battery lasts for about a week, and when it finally runs low, my low battery notifier sends a message to my phone through Pushover telling me to put it on its charger for a few hours.
To add to my step data, I got a Withings scale last year which logs my weight and BMI on Withings' website automatically every time I step on the scale. Fitbit's website syncs this data from Withings, so now I'm able to track my steps, flights of stairs, weight, and BMI, all automatically, all on Fitbit's website. I use this data mainly as a motivation to walk more and not get fat, just as my Wii Fit motivated me to exercise every day by tracking all of the data. When I know my Fitbit is counting my steps, I'll avoid hopping on the bus or train to get home and just walk. A few times I've left the house and upon noticing my Fitbit wasn't there, walked all the way back and got it just so the steps I was going to take that day would "count".