UPDATE (December 6, 2012): This app is now available for Android, too!
Solid Joys is a free application available from Desiring God (for iPhone and iPad) which provides a short daily devotional excerpt from the ministry of John Piper.
We've identified 365 of the best excerpts from Piper's printed books and online content which are able to stand well on their own when removed from their original context, and we've made sure to keep the excerpts relatively short.
The goal is that you'll be able to dive in for just a moment each day and come away with something solid to feed your joy in God. It's a fresh effort to apply the best of over 30 years of Piper's teaching to your everyday life and satisfaction in Jesus.
Download the app for free in the iTunes store. You can find it by searching for "John Piper" or "Desiring God or "Solid Joys."