I love the power of plaintext and texteditors. For adding more semantic meaning, I have experimented with the lightweight markup-languages txt2tags, emacs org-mode and Markdown. All three have their stengths and weaknesses. Txt2tags adds the ability to export to many different formats. Markdown is well supported on many editors on Mac OS an iOS. Org-Mode has so much power, calling it a markup-language is actually not doing it enough justice. If you are new to emacs, it takes a long time to learn it.
However, since txt2tags is the one I discovered very early, it is the format I have written many documents in. The Mac editor Textmate is a classic. In version 1.5 the txt2tags bundle is not included. But installing it is very easy.
Go to the Textmate-Bundles folder
cd ~/Library/Application\ Support/TextMate/Bundles/ If it does not exist, yet, simply create it with mkdir
Then get the current version of the bundle via git. If you don't have git installed, there are many ways to install it. I used macports.
git clone https://github.com/textmate/txt2tags.tmbundle.git
For nice highlighting of your headings, is like the textmate sunburst theme.
If you want to stay up to date with this bundle, simply run
git pull in the above mentioned folder.