Run Keeper is a free app.
The way it works:
Download the app.
Log onto the Run Keeper website and set up your account info. This is important, because the app can better calculate your calories burned during your run.
After you've done that, you are ready to go.
The app will sync with your music you have stored on your iPhone.
Start the app, then start running!
(One negative: There are times when I have to restart the app, because it seems to have some sort of malfunction with the GPS. I usually start the app, walk for 30 seconds so the GPS can locate me, then restart my run. This is not a big deal!)
As you run, the app calculates your distance and pace. It also calculates your calories burned. Also, it keeps up with your elevation. It's freaking awesome! Every five minutes, a nice lady comes on and tells you your time, pace, and distance.
This is your home screen. As you can see, you can select the type of activity you are completing. You can choose between a bunch of different exercises. Your playlist might be one you've created, or you can choose your entire iTunes library. I honestly don't know what the coaching thing does. The lady still tells you your time, distance, and pace even with this off.
All of your saved runs are kept on this screen. This is great, especially if you're training for a marathon.