An open source note taking app is one whose code is public, so anyone can check what it does with your notes. For most software that is a nice-to-have. For a notes app that promises privacy, it is the only way the promise can be checked at all: a closed app that says it is encrypted is asking you to take its word.
The six apps below all publish their code. They differ in almost everything else: which licence, whether encryption is on by default, whether notes are plain files, whether sync is yours to host, and whether the app is free. We build Dash and it is listed first, judged by the same rules as the others; its source is at github.com/Efesop/rich-text-editor.