[NightMKoder]

http://hackmd.io is by far my favorite markdown editor. It supports GH style markdown but with even more features: - Slideshow mode - Inline graphviz/MathJAX/mermaid and some other stuff - Collaborative GDocs-style editing (though no out of band comments) - Image upload - PDF export

It’s also somewhat open source (community edition is AGPL). You can host it behind a vpn for internal use.

Admittedly they’re not the most secure editor - e.g. image uploads are technically publicly accessible. Though supposedly there’s an enterprise version coming out with those features at some...point.

Disclaimer: I don’t work with these guys. Just a fanboy at this point. :)