I've recently started using a FitBit tracker to help motivate me to get more active in my life.
The simple little device clips onto my belt and is virtuously unnoticable during my day-to-day business.
Aside from being an extrememly well designed device, the companion iPhone app is equally thoughtful in execution.
Since FitBit is all about putting fun into staying healthy, there's a few nice touches along the way to remind you of that fact. My favourite is the empty state for logged activities showing a little dude lying down and drinking a beer.
Cute, motivational and effective.