LogSeq

https://logseq.com

From the Github README:

Logseq is a local-first, non-linear, outliner notebook for organizing and sharing your personal knowledge base.

Use it to organize your todo list, to write your journals, or to record your unique life.

The server will never store or analyze your private notes. Your data are plain text files and we currently support both Markdown and Emacs Org mode (more to be added soon).

In the unlikely event that the website is down or cannot be maintained, your data is, and will always be yours.

Logseq is hugely inspired by Roam Research, [[Org Mode]], [[TiddlyWiki]] and [[Workflowy]], hats off to all of them!

Not currently open source, although they say they will release it.

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