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).
Ranked in these QuestionsQuestion Ranking
Pro Automatic "table of content"
Automatically generates a "table of content" of your headlines.
Pro Dark and white theme
Pro Supports revisions
Allows you to go back in time to a previous version of your document.
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 Tag support
This is truly helpful for organizing your documents.
Pro Can create slide-shows
Supports creating slide-shows in markdown.
Pro Math support
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.