A hipster's choice in real-world notebooks. If you like Moleskine's leatherbound notepads, you may want to carry on with the brand even when on your phone. Though honestly, pen and paper are hard to beat when it comes to note-taking.
This one has a Mac version too, so you don't have to keep it all on your iPhone. That's great for jotting down a few quick thoughts on the road, and fleshing out your ideas later when in front of a real computer's full-sized keyboard.