The Substance Composer is a desktop application (OSX and Linux first, Windows later), where you create and manage digital documents. While traditional text editors usually deal with visual information (emulating its representations on paper), the Substance Composer is developed around the idea that content is data. Its structured document composition allows documents to be stored and represented in multiple arbitrary ways. On top of that, with its collaborative features you can annotate, comment and revise your content with anyone.
For docs see: http://interior.substance.io/modules/composer.html
To try an early version download: http://interior.substance.io/downloads/Substance.zip
To suggest a feature, report a bug, or general discussion: http://github.com/substance/composer/issues/