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Joplin is the best FOSS option I know of right now. It's a lifesaver. Sync it up to a bucket or something via webDAV and it's good to go.

Dendron is open source:

Doesn't Obsidian output open source (i.e., plain text Markdown documents)?

As far as I can see, there is no license attached to the output of Obsidian.

Open source is a license, not a standard.