https://www.marginnote.com/<br /> Note taking application with audio recording

https://www.literatureandlatte.com/scapple/overview

https://apps.apple.com/us/app/cardnotes-zettelkasten-notes/id1611515089

https://wordpress.org/plugins/slippy/

This WordPress plugin looks like it's a solid custom post type for creating a digital zettelkasten. Looks like it relies on tags/categories for linking, though has a custom link function. This in combination with Webmention could be a useful bi-directional link set up.