This is a great fitness app. Runners, Cyclists, Swimmers etc. RK will record your workouts, track your progress and gives you some neat features along the way. If you're a Fitbit user or have a Withings scale, it will collect that data as well.
Runkeeper is the best running app I've used so far. I particularly like the audio cues it gives you (e.g. every 1/2 mile it can tell you your average pace, split pace, distance covered, etc.) and the solid integration with the Runkeeper.com website. I'd recommend it to anyone who's looking for a great way to track their training runs.