Sure there are lots of good distraction-free text editors out there (Writeroom, OmmWriter) but it seems that there's already a pretty simple app in your basic MacOSX installation that can fill your needs: TextEdit.
Yeah I know, its default setup (rulers, rich text format) is not so attractive but if you find some time and tweak the preferences you can transform it to a simple, distraction-free text editor.
Here's how: Fire up TextEdit, open its Preferences (command + ,), select "Plain text" in Format group, uncheck "Show ruler" option and this is pretty much it! Restart TextEdit and behold, simple and clutter free interface! Plus, TextEdit has integrated spell & grammar checking and correction.
Bonus point: Try getting Consolas installed and set it as a default font for TextEdit, it really really helps.
Thanks Markp for the tip :)