That one app would be TextExpander, one of the most productive tools anyone who does a lot of typing can get. The idea behind it is incredibly simple, but it makes you so much more productive that once you've lived with it, it's hard to live without it.
What TextExpander does is allow you to create shortcuts for long or complicated text strings, scripts or formatted text with images. What you set up and how you set it up is completely your choice, and it's the easiest of processes to do.
You have a large box to enter what you want to shorten, what type of string it is, what you want to call it for organization purposes and what you want your shortcut to be. The most important thing you need to consider when you're setting up your shortcuts is that you don't use actual words because every time you type the key combination it'll do what its meant to do.
It'll expand into your snippet.
TextExpander runs in the background, and while there's obviously an app front to interact with and set it up, most of the time it's completely unseen, minimized to the task bar. You will also benefit from making sure it's one of the apps set to launch every time you boot your machine, otherwise you'll have to launch it for it to work. And if you ever need to turn it off, you can do this within the app rather than having to quit entirely.
What's good about TextExpander is that it's every bit as reliable on Windows as it always has been on Mac. It is also, right now, quite barebones. I'd like to see support for cloud backup and restore added in the future, as well as a way to export your snippet catalog to share with others.
My first experience with TextExpander on the Mac was by importing a whole bunch of snippets set up by some Mobile Nations colleagues, which was a huge help. Right now I'm having to set up from scratch again, rather than just open a backup from days gone.
But it's very much the tool I've been missing. I've tried to find a good alternative for Windows and there are some, but I've never been as happy as with TextExpander. It's still early days on Windows, and with a great developer behind it I'm fairly confident features will continue to come.
You can try it out for a week free of charge, otherwise it's about $4 a month if you pay monthly. But if you're a writer, give it a shot, it'll transform how you work.