A pad of adhesive notes for the back of your iPhone. Yes, there are apps for that. However, it's still easier to jot something down on a sticky note, and more satisfying to cross off an item with real ink. With Paperback, you can do a quick sketch or make a list the old-fashioned way, and slap it on the back of your phone. Use them one at a time, or put a small stack on the back of your phone to use continuously. Each page has a removable adhesive on most of the surface, keeping the note flat on your iPhone's underside.