WriteMonkey is a free, open-source, lightweight, and portable text editor designed specifically for writers. It provides a distraction-free environment and features such as typewriter sound effects, document statistics, a focus mode that dims all but the current line, and more. It is highly customizable and can be used as a basic text editor with support for popular markup languages such as HTML and Markdown, or it can be used to save and organize ideas, stories, and projects. It also includes a variety of text formatting options and support for multiple languages. WriteMonkey is available for Windows, Mac, and Linux.
FocusWriter has similar window customization, such as background/foreground color, font size and type selection and size. It's also portable and it's also free. There are not as many features as WriteMonkey, but the distraction-free environment is the same.