Daylio - Daylio enables you to keep a private diary without having to type a single line.
Future Me - Send an email to your future self
Penzu - Keep all of your thoughts in one place using Penzu. The app is similar to a journal that you might write in but with a few modern touches that allow you to do everything from sending messages to decorating the pages.
Letter to Yourself - Send a physical letter to your future self
Journey - A diary that keeps your private memories forever.
Postpone Pixel - Send a photo to yourself or a friend in the future