A free, open source, offline-first spaced repetition system that has first class support for collaboration, curation, and plugins. It's Reddit for flashcards.
Pentive has an extremely powerful plugin system. There is literally no limit to modding the UI. Any default UI component (I'm using SolidJS) may be overwritten with a custom element. See example plugins.
If you're technically inclined, see design decisions for a review of Pentive's architecture and design. Contributions welcome!