Cerveau

Cerveau is a Zettelkastesten-style note-taking app, based on plain-text Markdown files stored in a Git repo, with two-way sync. Edit your notes in Cerveau, and then later edit them on the text editor of your choice, and then back again to Cerveau.

In short, Cerveau gives you the best of both worlds -- future-proof plain-text notes, and a convenient web app for viewing and editing them.