As I mentioned the usage of Markdown for the Slate documentation builder, here some Markdown editors:
ULYSSES FOR MAC/ IPAD
Highly customizable editor.
STACKEDIT INBROWSER SOLUTION
Use it anywhere and sync to dropbox if you like.
MARKDOWN PAD FOR WINDOWS
Nice split-screen setup, that shows you how the html conversion looks like.
MARKMYWORDS FOR LINUX
A nice minimal editor that was developed for Elementary OS.