Note-taking apps differ less in what they store and more in who controls the data.
Wiki and knowledge base
If you currently rely on Notion and want more control over data, lower long-term costs, or a open-source licensing model, Outline covers most of the same ground.
Most open-source alternatives require more setup than their commercial counterparts and may lag on polish, mobile apps, or ecosystem integrations. Check the project’s documentation for current feature parity with Notion before committing.
This recommendation is indexed from awesome-oss-alternatives. Read their full project notes for the latest changes.