Second only to Caffeine, this may be the smallest yet most useful piece of software I own.

TextExpander does exactly that. You can register snippets of text and it will expand them into longer, intelligent or interactive snippers of text.

A simple example. When I need to type in my email address @@h becomes my full home email address, @@w becomes my full work email address. dts prints the current date like this 2013-06-06.

My absolute favorite, at least before the GitHub team became even smarter and allowed direct copy & paste of images into comment forms (try it, it's nuts), was my Markdown Image snippet. With an image URL inside my clipboard, I can simply type mdii and it becomes ![%|](%clipboard). The %clipboard denotes where the clipboard contents will be pasted while the %| denotes where my cursor will be moved so I can start typing a description for the image.

If you ever type the same information more than 3 times a day, you should be using TextExpander.