- Mobiledoc - https://github.com/bustle/mobiledoc-kit - framework to build editors with a standardized JSON structure
- Substance - http://substance.io - library for building editors and publishing systems
- ShareDB - https://github.com/share/sharedb - framework to sync any JSON document using operational transforms, add real-time collaborative editing to anything else
These use separate document structures instead of HTML, some are more modular libraries than full editors
- ProseMirror - http://prosemirror.net - supports collaborative editing, offers similar options to Mobiledoc for data structure