I just remembered something. It's not really a fitness app, but for gamers, it might be a good additional incentive to workout. It's called Habitica, and it's basically an app where you can write down your daily chores, big tasks, and habits you want to learn and bad habits you want to get rid off.
Completing the tasks you wrote down will give your avatar XP, money, mana, and if you're on a quest you deal damage to the monster you're trying to kill.
Again, it's not a workout app, but it can give you a good incentive to workout. There's also guilds where you can talk to other people about a subject. If you join a workout guild (you can join as many as you want, by the way) you can talk about your workout strategies with others and others will help you as well. Habitica's community is really great, too. You should check it out.