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
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 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.