Requires iOS 7
Brevity allows you to respond to text messages quickly and easily. You set your desired shortcut, and when nothing's typed, it takes the place of the grayed-out send button. With one touch, your shortcut's sent.
When you want to send a standard message, just go ahead and start typing. The send button fades right back in, and you're ready to go.
Your shortcut can be anything, from plain text to emoji, or even unicode characters. Changing your shortcut takes one quick trip to a preferences pane, and does not require a respring.