
Ghostwriter: HTML preview only (non editable) but also have a nice outline panel and syntax highlight. Also can import-export to several formats via pandoc integration. And not only export to HTML, ODT and PDF. The "source code mode" have syntax highlight, including bigger fonts for headings and italics for emphasis.

Although it is not FOSS, is perfect as markdown viewer because it work by default nearly as a WYSIWYG editor ("live preview mode") with a Outline panel (table of contents) that is very convenient for large files. Typora: It is still in beta phase, but it’s free meanwhile (it’s not clear how will be licensed the stable version). Some in official repositories, others not, and each with their own strengths and weaknesses, but as suitable viewers I would like to highlight these:

There are a few more free markdown editors with preview available for *nix systems. However, if you need see the file of ReText from a terminal, one option could be convert the marddown to html with pandoc and see the html copy in lynx: pandoc file.mkd > file.html lynx file.html

Is a GUI program, but I find useful for this task ReText, that is an editor for Markdown and reStructuredText with a preview mode:
