I wonder that there seems to be no text editor on the mac installed that is capable of saving just text without any formatting. Which text-only editors do you know that allow for editing text without any formatting like font sizes etc.?
emacs
, vim
, nano
Mac OS X TextEdit supports editing text files without formatting.
You probably would consider reading this http://hackresponsibly.com/the-best-text-code-editors-for-windows-mac-os-x-and-linux/
If you install and use either of Vim or Emacs, you will have a text editor that can be used on any major platform now and in the future, and most of the minor ones too.
As a bonus, they are both very well supported, very powerful, and (not surprisingly) have existing, rich support for just about any kind of text editing you might want to do (and even the types you don't yet realise you want).
Of course, they both support creating plain text files by default.
For free Maclike (for given values thereof) GUI ones, there's Komodo Edit, Smultron, TextWrangler.
From the Emacs variants, my personal favourite is Aquamacs, because it tries its darnedest to mitigate the OS X vs. Emacs impedance mismatch, and comes with some of the hairier packages preinstalled.
It was a couple years ago, but I tried everything I could find for doing front-end editing (JS, XHTML, CSS, etc). I found Komodo to be the best, most comfortable, most intuitive. It's not perfect, but it was the closest I found.
It sounds like you aren't aware that it's possible to save plain text files with TextEdit. While the default is RTF, you can go to Format > Make Plain Text to switch.
And of course Xcode will save plain text files, and let you control things like encoding and line endings.