iOS: We're fans of the CARROT apps, which include a to-do list and alarm clock, because they manage to make unwelcome tasks a little more fun by shaming you into getting things done. CARROT Fit does the same thing for weight tracking.
At its core, CARROT Fit is really nothing more than an app where you track your weight. Each day, you enter in your weight and as you lose it, you're rewarded with in-game points (and threats if you gain anything). CARROT Fit logs you weight and shows you a graph that tracks your losses, but otherwise it's a pretty straightforward weight tracker. Except for the AI that harasses you around every turn anyway.
CARROT Fit ($1.99) [iTunes App Store]