I've used the Nike+ RunClub App for years now and the UX and benefits continue to improve with each release.
Track runs, unlock achievements, keep an eye on how many miles you've logged on your running shoes, follow a training program, or engage in a single run.
Perhaps my favourite feature is the integration with Facebook that allows people to "cheer" for you during a long run. There is nothing better then your friends sending cheers through during your 13th KM before 8am.