Journaling app with gorgeous interface and impressive list of features that will finally get you to journal. For those shying away from the 10 year old girl dear journal image, here's a list of interesting ways of using the app, like documenting your next vacation trip, tracking your weight loss goals, or annotating ideas for your next project.
Mindmapping is easily the best thing you can do to put yourself into big picture thinking, and this app is simple and beautiful enough to allow you to focus on your thoughts instead of the app itself.
This is the one app that I don't have yet but will jump immediately for it. It's a camera app for your iPhone that allows you to fiddle with the manual controls (ISO, aperture, shutter speed, white balance, etc). If you ever wanted to familiarize yourself with the basic concepts of photography, this is probably the cheapest way to experiment and learn.