Perhaps the first thing you learn in David Allen's Getting Things Done is that your brain is an awful place to store things. Your brain is a computer, not a hard-drive.
Because of this, we often find ourselves missing appointments, forgetting assignments, and spend our days in a haze of ambiguity regarding what our next tasks are. That's where Due comes in.
Due is an app for iOS that operates somewhat like the alarm function of the integrated "Clock" app, but is far more user friendly and obnoxious. You might ask yourself why someone would want an obnoxious reminder app. For those of us who are prone to ignore our reminder alarms, this app pesters you every minute or so, provoking you to actually get your work done.
The app allows you to easily control:
For people who are stubborn when it comes to reminders, I cannot recommend this app enough. It cost $4.99 in the app store, but is worth every penny.