A free, open source, local-first, spaced repetition system that works offline, has p2p syncing, plugins, and first class support for collaboration. It's GitHub/Reddit for flashcards.
Pentive's plugin system allows developers to directly modify the dependency injection container. Any UI component may be overwritten with a function that returns any DOM node. See example plugins.
YouTube demo:
If you're technically inclined, see design decisions for a review of Pentive's architecture and design. Contributions welcome!