This week ONLYOFFICE announced the release of the new version of its desktop editors that allows users to edit documents, spreadsheets, and presentations offline and collaborate on documents switching to the online mode.
First, ONLYOFFICE opens the source code of its editors under AGPL v.3. Second, users can extend the editor functionality with integrated add-ons or create their own add-ons. More here: http://www.onlyoffice.com/blog/2016/10/onlyoffice-desktop-editors-go-open-source/