Some alternatives not mentioned here that I am aware of are Mnemosyne Proj [1] and org-drill [2]. I have only ever used anki though. I am going to try org-drill because anki has over 40 dependencies I would not need to compile if I just switched to org-drill. The one nice thing about anki is the community cards you can download, but at the same time I feel that an important part of learning is making the cards yourself.

[1]: https://mnemosyne-proj.org/features

[2]: https://orgmode.org/worg/org-contrib/org-drill.html/