iOS: You'll find a lot of different activity trackers out there, but they're not really useful unless they actually push you to exercise more. Human is an app that tracks your data and pushes you to get out and move if you don't meet a simple 30-minute goal.
Human has been around for a little while, but a recent update makes it work a lot better than it did originally. Human's basic purpose is pretty simple: it automatically tracks everything you do, from moving to walking to running, and pushes you to exercise for at least 30 minutes every day. It then keeps track of your movement to make sure you're hitting that every day and maintaining a streak. It works well and since it tracks more than just steps, you get a more accurate of an idea of how much activity you're actually doing every day than you would from a pedometer.
Human (Free) [iTunes App Store]