iOS: Day One is a beautiful, highly useful app for Mac and iOS that encourages daily writing. As part of a special sale celebrating the App Store's 5-year anniversary, you can grab the iPhone/iPad version for free (it's normally $1.99).
Among the many features: customisable reminders to record something every day (or whatever period you choose), calendar and yearly views for a timeline of your entries, password lock, photo entries, tagging, exporting to PDF, and a lot more. As full-featured as the app is, it's also very simple to use. Plus, if you have the Mac version, Day One can sync via iCloud or Dropbox to the desktop.
If you've ever wanted to make writing or journalling a daily habit, grab Day One while it's free.
Day One (Free) [iTunes App Store]