It's Q10. Unfortunately it's for Windows only.
It shows a black full screen and amber text that's dimmed down enough not to blind you when large chunks are on the screen at the same time (you can switch to any color). There are no distracting toolbars, no endless rows of icons that you never click on. It does have all the basic functions you'll need (in mapped key form) - cutting, pasting, saving, find, undo, redo, spell check, etc. It has typewriter sound effects to provide auditory feedback while you type (you can switch it off).
Most importantly, though: no What You See Is What You Get. Formatting, layout, etc., can come later, if they need to at all. If you're not going to be sending or handing it over to somebody, why bother with all that pointless polish? The mind can be in either of two modes, but not in both, during the same writing session — producing language, and preparing it for presentation to others. Substance or style.
Later, you can still open the file in a program meant for formatting. Just about everything reads .txt files.
If it's been awhile since you've used a word processor whose basic features just let you be, rather than interrupting the flow of thought-recording, you'll be surprised how easy it is to put your thoughts down.
At the office, I've started using Vim since you can configure more of the default settings and pair it up with LaTeX for formatting equations and references in journal articles. But when I'm done playing around with its settings, for most things it's going to behave like the one you can quickly and freely download, and have running in no time. I wouldn't mind if Q10 were the only word processor that I used from now on. It's such a breath of fresh air in our technologically stultifying society.
Related post: light-on-dark color schemes to boost productivity.