Open Source Markdown Editor
Markdown's whole appeal is that the file stays plain text you can read anywhere, so the real question for an editor isn't features but how faithfully it round-trips your source - whether export adds invisible cruft, mangles tables, or quietly rewrites your syntax. The open source editors here treat the .md file as the source of truth rather than a database export, render a live preview without trapping your writing in a proprietary container, and let you push the same text to a site, a PDF, or a repo unchanged.
MarkText
Open-source Markdown editor with real-time preview and live rendering
Joplin
Offline-first Markdown note and to-do app with encrypted sync across desktop and mobile
SiYuan
Privacy-first personal knowledge management with Markdown WYSIWYG, block references, and Docker hosting
Zettlr
Markdown publication workbench for researchers, journalists, and writers with citations and Pandoc exports
ghostwriter
Distraction-free Markdown editor for Windows and Linux with live HTML preview and export processors
ReText
Editor for Markdown, reStructuredText, Textile, AsciiDoc, and custom markup languages
Marker
GTK+3 Markdown editor for Linux with scientific syntax, math, diagrams, and Pandoc exports
Vivliostyle
Open source CSS typesetting engine for HTML, EPUB, and web publications