What are the best Markdown previewers?

Duplicate Pro

From what I can gather, all options currently listed, except for Marked, are actually editors, not previewers. Previewers allow using any editor.

I'll move the answers to the appropriate places:
http://www.slant.co/topics/899/~what-are-the-best-markdown-editors-for-os-x
http://www.slant.co/topics/1852/~what-are-the-best-markdown-editors-for-windows
http://www.slant.co/topics/699/~what-are-the-best-wysiwyg-editors-for-markdown