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Marijn Haverbeke, creator of the in-browser code editor CodeMirror that we use here on CodePen, has created a new in-browser editor, this time for Markdown. It's called ProseMirror:

Several companies have implemented systems that improve the situation. Google Docs is probably the most awe-inspiring browser-based document-editing interface around, but you can't just drop that into your CMS. Medium has built a very nice lightweight editor custom-tailored for their blog-editing use case (their blog post about it is somewhat relevant to this project). But again, closed source.
ProseMirror is something in-between those two: a lightweight, extendable, general-purpose document editor with support for collaboration.

It's written, but it only goes MIT open-source if it gets fully funded on Indiegogo.

Count us as funders!