Obsidian

For people who want their notes to outlive any app, this one included. Obsidian stores everything as Markdown plain text files in an ordinary folder on your own device, so if the company vanished tomorrow, any text editor could still open your notes. Links between notes, a graph view and a huge plugin library make it the default choice for building a personal wiki. The app is free, commercial use included, and Obsidian Sync costs $4 a month billed annually if you want its own encrypted syncing. The real risk is not lock-in but tinkering: plugins and themes can quietly become the hobby instead of the notes.