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Outline
The fastest knowledge base for growing teams. Beautiful, realtime collaborative, feature packed, and markdown compatible.
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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
Joplin or Trillium are the usual go to solutions.. The most "OneNote" likeness, is probably Outline.
The one I am pretty excited about, is NotesNook, unfortunately whilst the server is open-source, its currently not self-hostable, but from what I gather, they are working on this... I love the way they are/have openned this up, and will probably sponsor it once its available.