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Can we make that 6? Newly launched Flaab is perhaps the simplest weight loss site we've seen: enter how much you want to lose and place a monetary bet. If you fail to lose (the weight), you promise to send the money to your worst enemy, a rich friend, or your ex.
No, we won't be adding this to the list: it doesn't pass the "useful" test in our eyes. But it is a fun distraction, if a tiny bit gimmicky. What do you think: is a promise made on a website something you'll stick to, or immediately forget?
Or perhaps you prefer one of these 25+ Web Apps for Dieting and Getting in Shape?