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.
Google Open Source - All of Googles open source projects under a single umbrella
Boostnote - Boostnote is an open-source note-taking app.
Code NASA - 253 NASA open source software projects
Standard Notes - A safe place for your notes, thoughts, and life's work
Disney Open Source - Explore Disney's Open Source projects