GymPact is a highly useful iPhone/Android app that motivates users to healthier lives by providing a financial incentive to working out. The app itself is free, but there is a potential cost (or profit!) involved. You make a pact of the number of times you'll go to the gym per week as well as set a financial penalty if you break your pact. The app provides a financial incentive by rewarding you for going to the gym ($0.50-0.75 per workout) but also penalizes you with your fine if you break your pact that week. The app works with your PayPal account for the financial transactions.
To read a review of the app by a physician who's used it: A physician's one year review of GymPact, an app that pays you to go to the gym
To get the app in the iTunes store:GymPact - Cash reward to motivate gym and RunKeeper workouts
To get the app for Android: GYMPACT. Commit. Workout. Earn $$$. Paid for by non-exercisers!
GymPact Official website: GymPact/Never miss another workout