Markdown has become an essential file format, especially if you are sharing source code on Github or Bitbucket. ReText is a good Markdown editor for Ubuntu. It can be installed easily:

$ sudo apt install retext

I like to use ReText in the live preview mode where the resulting HTML is shown in a pane on the right. So, it acts as an excellent Markdown viewer too. I have no idea why this is not turned on by default.

Tried with: ReText 4.1.2 and Ubuntu 14.04

MarkdownPad: A markdown editor for Windows

The markdown format is increasingly used at websites like GitHub. Thus, there is a need for a simple markdown editor that can be used to edit and view markdown files.

MarkdownPad is a markdown editor for Windows which I found does this job well. Just download and install. Associate .md files with MarkdownPad, which can be found as MarkdownPad.appref-ms in the directory %USERPROFILE%\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\MarkdownPad

Tried with: MarkdownPad and Windows 7 Professional x64