This was written in Dark Room.
(It's a text editor that allows you to edit text. And not worry about other troublesome details.)
Dark Room is, along with it's associated brethren/ancestors, perfect for removing distractions from the writing process. My system tray notification area is always blinking and clogged; my desktop is bright and gaudy; I always feel I need to multi-task. These problems disappear behind the black expanse of a Dark Room window.
There are no menus or title bars, no spell check (so, no annoying squiggly lines), no formatting. Nothing to get between the writer and the written.
There's always Notepad, if you want to write notes. And there are various advanced forms of Notepad for programming and technical tasks (ConTEXT, TextEdit). Just think of Dark Room as being a form of Notepad entirely dedicated to creative tasks.
Of course, such things are only neccessary if you're stuck in Windows or Mac OS. If you're using Linux, a plain, 80-character-width console vim
window is always just a few keystrokes away. But if you can't be bothered, I recommend Dark Room.