[gravity13]

The closest is ckeditor, I think. But that's still a long shot away.

Notion is actually really well constructed (as somebody who's been in the space building tools kinda like it for a while, it's quite impressive). But I agree, the space needs an open source notion...

[lobster_johnson]

Seen ProseMirror? http://prosemirror.net/index.html

[TuringTest]

Yes, but it's currently just a rich text editor, it lacks the outlining capabilities (i.e. creating separate hierarchical pages, and tagging). These should be relatively easy to add, though.