QOwnNotes is the open source (GPL) plain-text file notepad with markdown support and todo list manager for GNU/Linux, Mac OS X and Windows by Patrizio Bekerle (pbek on GitHub and IRC), that (optionally) works together with the notes application of ownCloud or Nextcloud.

Via http://www.webupd8.org/2016/09/qownnotes-is-note-taking-and-todo-list.html