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I use sublime text 3 with following 2 plugins - Markdown Preview, for previewing in browser - Markdown Editing, great editing environment

I don't prefer online editors as don't like to see the live preview while I'm typing. But here's a good one dillinger

I can second the Sublime Text recommendation and add MarkdownPad (http://markdownpad.com/) for Windows.

I tried using Sublime text 3 for markdown but there was no significant syntax highlighting.