If your priority is real-time collaboration + Markdown as the actual document format, rather than exporting from a proprietary editor, these are strong options:
hedgedoc.org — Probably the cleanest fit. It is open source, self-hostable, and built specifically around collaborative Markdown. Multiple people can edit simultaneously, with live preview.
hackmd.io — A polished hosted collaborative Markdown editor. Its current product documentation says Markdown is authored, stored, versioned, and served as Markdown rather than merely being an export format.
.app — Particularly interesting if you already have a folder of files. It works directly with local Markdown files, supports real-time collaboration, and is open source/self-hostable.
usemdocs.com — A newer option built around plain Markdown, with Google-Docs-style multiplayer editing plus a Git-like CLI workflow. It explicitly describes Markdown as the lossless source of truth.
getoutline.com — More of a collaborative knowledge base than a pure Markdown editor, but it supports real-time collaboration and Markdown export/copy. Its documents aren't exclusively stored as .md files, so it's a weaker fit if avoiding proprietary storage is your central requirement.
My practical shortlist
If you want…
Consider
Self-hosted, mature Markdown collaboration
HedgeDoc
Best polished hosted experience
HackMD
Edit an existing Markdown folder directly
CollabMD
Markdown + Git/CLI workflow
mdocs
Wiki/knowledge-base features alongside Markdown
Outline
If by “does not lock me into a proprietary format” you mean “I want the canonical files to remain ordinary .md files on my filesystem/Git repo”, I'd narrow the field to CollabMD, mdocs, and HedgeDoc, with the first two especially aligned with that requirement.
If you want real-time, multi-user collaboration , native Markdown support, and absolute freedom from proprietary lock-in (meaning your data is stored as plain .md text files that you can host or export anytime), the best options are fully open-source and self-hostable tools.
Here are the top recommendations that fit your criteria:
HedgeDoc (formerly CodiMD / originally tied to HackMD's open-source core) is arguably the best match. It is an open-source, web-based, self-hosted collaborative Markdown editor. Multiple people can write and edit a document simultaneously in real-time via a split-pane view (code on one side, rendered output on the other). It saves data in plain Markdown using HedgeDoc Flavored Markdown , and you can easily import/export standard .md files or even turn notes into slide decks. It runs cleanly via Docker.
Etherpad with the ep_markdown plugin is a classic, ultra-fast real-time collaborative text editor. While standard Etherpad is purely operational-transformation plain text, adding the Markdown plugin allows you to view, import, and export your collaborative pads directly as Markdown. It is entirely open-source with zero proprietary formats, though its live editing is WYSIWYG/stylistic rather than "type-the-syntax-as-you-go".
SilverBullet is a newer, open-source, local-first and self-hosted collaborative space. It treats your entire workspace as a collection of plain Markdown files stored right on a server or your local machine, allowing real-time sync and page editing across clients without locking you into a proprietary cloud or database structure.
Would you prefer a browser-based tool you can self-host (like HedgeDoc via Docker), or are you looking for something that integrates into a local folder/Git workflow?
Yes. My top recommendation is MkDocs if your priority is keeping documentation in a genuinely open, portable format.
🥇 MkDocs — best fit
MkDocs stores your source documentation as ordinary .md files in a directory, typically alongside a small mkdocs.yml configuration file. It generates a static website from those files.
Why I like it for a team:
Markdown files are the source of truth — no proprietary database or document format.
Works naturally with Git/GitHub/GitLab, so changes can go through pull requests and version history.
The resulting site is static HTML, so you can host it essentially anywhere.
Open-source, BSD-2-Clause licensed.
Lots of themes/plugins, including the popular Material for MkDocs ecosystem.
🥈 Docusaurus — better for a polished documentation portal
Docusaurus is another excellent choice. It uses Markdown/MDX files, supports hierarchical docs, versioning, localization, search integrations, and generates a static site.
The tradeoff is that Docusaurus is more of a documentation website framework than a simple team wiki. It also uses MDX, which adds capabilities beyond standard Markdown.
For most engineering/product teams, I'd start with MkDocs + Git. Your actual knowledge remains a folder of Markdown files, meaning you can move to another renderer—or simply read/edit the files directly—without having to migrate out of a proprietary database.
If you tell me whether your team is technical or non-technical, I can also recommend a few open-source wiki tools with a browser editor that still keep the underlying content portable.
