Procyon is a free and open source note keeping or note taking application for Linux, macOS and Windows. It supports plain text and markdown for notes. You can view your current notes in preview mode. It is still in early stage and you can use it as a your simple to user personal note taking app. It is using single SQLite database to store your notes. It you want to sync function, then you need to use Dropbox or Google Drive to sync your notes. It also comes with syntax highlighting and custom highlighter for your notes.