I second the use of Cyclemeter. I also tried all the popular, and some not so popular, iPhone apps and Cyclemeter stood out over all of them. I've yet to see one that has so much customization and data statistics as Cyclemeter. I don't think you can go wrong. Plus, if you opt to get the Elite version, you pay once a year rather than once a month for the paid versions of the other popular cycling apps. As mentioned, the only truly free on is Wahoo Fitness but it's very limited on features compared to other cycling apps.