iOS: Most apps that use social support to help you reach your goals encourage your friends to bet on your success. Betchyu on the other hand asks your friends to bet against you, and encourages you to prove them wrong. It's a little sadistic, but if you're competitive, it could work.
When we say "bet", we are talking about money — or at least, money in the form of gift cards. You select a goal you'd like to work towards, either losing weight, quitting smoking, general exercise, or running. Add the specifics of what you want to do, and select a gift card that you and your friend will wager over. Invite your friends to bet against you, and if you stick to your guns and hit your goal, you win the gift card (and your money back.) If you lose, your friend gets the gift card instead (which, ideally, you could ask them to share with you.)
All in all, it's another "put your money where your mouth is" approach to goal-setting, as we've seen with apps like previously mentioned Pact, the ever-snarky Go Fucking Do It, and the data-heavyBeeminder, to name a few. Hit the link below to learn more. (It's iOS-only right now, but an Android version is promised.)