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