HackMD is a clean but powerful online interface for developing markdown documents. It is collaborative (multiple editors and viewer). It renders expected output real-time, and it is free. They are also beginning to offer an enterprise edition with additional enterprisy features (super refined permissions, auditable changes, etc).
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Allows multiple people to edit the same note in realtime.
It's multi-platform (works well on smartphones) and there is no need to install anything.
Pro Automatic "table of content"
Automatically generates a "table of content" of your headlines.
Pro Supports revisions
Allows you to go back in time to a previous version of your document.
Pro Math support
Use LaTeX seamlessly within the editor.
Pro Tag support
This is truly helpful for organizing your documents.
Pro Edit and view-only permission settings
Note: Enterprise Edition has even more refined permissions, but the permissions in the non-enterprise edition are pretty powerful.
Pro Dark and white theme
Perfect for working at night.
Pro Can create slide-shows
Supports creating slide-shows in markdown.
Pro Book mode view
This is really helpful for longer documents. Or even "books." This is also super helpful for journaling activities and the like.
Pro Google Drive import and export
Pro Shortcuts from Sublime Text, Emacs and Vim
Pro Github Gist import and export
Pro Dropbox import and export
Con Not as beautiful as StackEdit
Con Slow servers / problem with online / offline switching
Servers can be slow / down for a few seconds preventing from working on the documents currently being edited.
Con Custom grammar and spell checker
It does not work well in French (and probably other languages).
Con Tries to be smart
It guesses what you want to do and tries to help, often in a wrong and annoying way, for example it when working with list items. Ironically Markdown was built to solve those problems (you format yourself with simple codes and it just works).
Con Cannot organize notes
Doesn't have the ability to organize notes or create notebooks.