https://github.com/burtonator/polar-bookshelf https://github.com/Yvee1/hascard https://github.com/amy6/delern

and anki

https://mnemosyne-proj.org/ - The Mnemosyne Project

I'm not sure that there's a dozen, but one very well-known one is the third word of the post title. :)

Leitner Box. Syncing kind of sucks though.

https://en.wikipedia.org/wiki/Leitner_system

Tiddlywiki is Open Source and has an Anki plugin (TiddlyRemember)

org-mode with org-fc for spaced repetition