Sublime Text is a sophisticated text editor for code, html and prose - any kind of text file. You'll love the slick user interface and extraordinary features.
Vim ("Vi IMproved") is an advanced text editor that allows syntax highlighting, word completion and has a huge amount of contributed content. It also has a GUI version called GVim.
A markdown editor, markdown reader.
Powerful knowledge base that works on top of a local folder of plain text Markdown files.
Pandoc is a Haskell library for converting from one markup format to another, and a command-line tool that uses this library.