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Another interesting library built for collaborative editing is Prosemirror. It's also in its infancy stages.

It's by the guy who created CodeMirror and TernJS.

Here's a direct link to ProseMirror's collaborative editing demo http://prosemirror.net/demo_collab.html#edit-Example (which is up, and fast)

I'm definitely excited for ProseMirror. If it's as quality as CodeMirror I'll be happily integrating it into everything I build!