Later for iOS (free) is one of those apps I thought about inventing myself. It's a simple app that sends you a notification reminding you to send a text, email or tweet at the appropriate time. You may have thought of something you want to message someone about, but it's after midnight, and you want to be reminded to send it in the morning. That's what this app does.
The app is easy to use. Just choose a recipient and set a time for the reminder to appear. When it does, just tap it and you'll get a pre-addressed tweet, text message or mail message. Fill in the message and off it goes.
Of course even better would be the ability to actually write the message and have it go at a preset time. According to the developer, that is coming.
Later for iOS
You could use Apple's built-in Reminders to accomplish something similar, but the message wouldn't be pre-adresssed. Later has been useful to me, and it is simple and elegant. When it adds the ability to actually send off the message, it will be a killer app, and the sort of thing Apple should build into iOS 7.
Later is not a universal app, and it requires iOS 7. It is optimized for the iPhone 5.