Hey! ReadMe.io founder here. We custom built ours, because we couldn't find anything else on the market that worked the way we wanted.

The closest I've found otherwise is ContendBuilder.js:

http://www.innovastudio.com/content-builder.aspx

SirTrevor is similar, but without drag+drop:

http://madebymany.github.io/sir-trevor-js/example.html

For us, it was built using two linked jQuery Sortable lists (the sidebar, and the actual content). It's easier than it sounds, especially with Angular :)

For "WYSIWYG" content editing, ProseMirror looks like the best implementation so far [1]. Pure JS editor without the buggy contenteditable stuff. I'm sure that it can be extended to support other "rich" objects such as todo lists.

[1] http://prosemirror.net/index.html

Substance http://substance.io/ is well worth taking a look at.

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...

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

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.