Logseq is pretty similar to Obsidian, and it’s FOSS. It’s still really young, but I’ve enjoyed using it more than Obsidian for my personal note taking style. It’s block-based and focused on daily journals, so instead of folders of individual notes the tags/references become interlinked pages. It’s been cool to see my daily logs become a web of concepts. Syncing is a new function they’re adding for supporters, but it can be done with Syncthing if you’re nasty.

It’s definitely a different way of note taking than Keep or Joplin and maybe not for everyone, but I hope I’m at least doing it justice and piqued someone’s curiosity!