I joined a few days ago and loving it! It's childish I know but I get a kick out of points and badges!
For anyone that hasn't heard of it - You set up a profile and then every time you exercise you log it and you receive points, earn a certain amount of points and you "level up" to the next level. You earn achievements for things like running a certain distance in one session and you can take on quests to try new exercises/challenges. It's all social too so you can join groups and challenge people to duels (lol).
It's like turning exercise into a game and it's a tad geeky but really motivating if you like that sort of thing.