I have been using this one for a while. It's a Markdown editor, which has a live preview option. If you choose not to have this, its a plain text window.
No fancy extras, just simple.
For the moment its free as its in beta, so you have nothing to lose.
*comes walking into the thread with a Space Shuttle Thermal Tile suit*
The best editor is nano. It's simple, and it is deadly.
And actually you don't need to download either. Recent-ish vim, nano, and emacs all come preinstalled on OS X.
I actually do use nano primarily for writing software. I use VIM when I need the superior regex support or multi-line editing. Otherwise I'm a lot faster with nano.