This markdown editor/previewer is the best one so far for my purposes:

• has editing and preview in a split view: see content in realtime
• handles technical text (lots of code snippets)
• has the following extensions (via discount):
  • Smartipants substitutions
  • specify image sizes
  • definition lists
  • alpha lists
  • tables like PHP Markdown-Extra
• it’s free

Link: rentzsch/markdownlive via github.com