The workout app. I usually run on an indoor track. Ran with the Apple Watch yesterday and it guided me though the whole workout so I knew how long I'd been running, how far I'd gone, my pace, heart rate, etc. Very useful information. One mile is 9 laps around the track, and I'd almost always lose track of lap count, leaving me guessing. But with the Apple Watch, I can flick my wrist and see exactly how many miles I've run. So cool. Definitely will change the way I exercise.