Windows/Mac/Linux: FocusWriter is a lightweight, portable and feature-=rich word processor with an emphasis on distraction-free writing. You can skin it, set writing goals, and enjoy features not usually found in similar writing applications like spell checking and tabbed documents.
Most "distraction-free" writing tools are focused on cutting everything out until you're left with little more than a glorified copy of Notepad. FocusWriter offers features usually absent from focus-centred writing tools like spellcheck, tabbed documents, themes, real-time stats on your work (character, word, paragraph and page count), as well as daily goals based on word count or time spent writing. The interface is there when you want it and vanishes into the background when you just want to write.
FocusWriter is freeware, portable (see the readme file for notes on how to make it portable for your OS), and available for Windows, Mac and Linux operating systems.