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.
Dillinger - joemccann has 95 repositories available. Follow their code on GitHub.
Boostnote - Boostnote is an open-source note-taking app.
Typora - A minimal Markdown reading & writing app.
Standard Notes - A safe place for your notes, thoughts, and life's work
Caret - Better Markdown Editor for Mac / Windows / Linux