Joplin - Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, tagged and modified either from the applications directly or from your own text editor.
Manuskript - Open-source tool for writers.
Boostnote - Boostnote is an open-source note-taking app.
StoryMill - Developers and publishers of Mac, Win, and iOS productivity, home and office and writing software such as MacGourmet, Paperless and many more. Based in MN.
Standard Notes - A safe place for your notes, thoughts, and life's work
Novelize - Novelize is an online novel writing app designed to keep authors on track.