In January 2014 ... MyFitnessPal announced they were partnering with GymPact - now simply called Pact, as there were food pacts as well as exercise. At this point, I also saw that Fitbit was now an accepted partner, with 10,000 steps equivalent to a "workout" ... so I joined, and you can see my review here (Pacts and Paydays).
Another very popular possible reward/challenge is DietBet. I have not personally participated in one of these, but I know of MANY people who have. Again, it is a little more involved then simply syncing my Fitbit. It does require an upfront payment, and pictures/proof of weight loss success. Like GymPact, losing money is as much of an incentive as winning money.