There are a good selection of other "minimalist" programs around on all platforms (WriteRoom, Q10, Darkroom, PyWriter, etc.), but Focuswriter has the distinction of being cross-platform: it is available for the Mac, Windows, and Linux. It is free, and open source.
I looked at it on the Mac. What I like about it:
- plain text - but saves to other formats.
- works with Dropbox
- “distraction free” environment, meaning full screen
- themes (allowing one to change background, including putting an image on the screen, and fiddle with text fonts, sizes, and color)
- word count
- spell check
- multiple files can be edited at once
- a “focus mode” that highlights either the current paragraph or sentence
There are advantages to having a program that looks and works the same on all your desktops. However, Focuswriter doesn't exist on the iPad or Android.