I'm surprised no one mentioned Memoet (https://github.com/memoetapp/memoet). It is a much more modern version of Anki - much more open algorithm, REST APIs and universal interfaces and self-hosted control over one's data.
I'm building https://github.com/AlexErrant/Pentive/ which is basically Anki + Github + Reddit; people can optionally upload their cards for others to download/fork, and the most popular cards rise to the top. It's FLOSS, offline-first, supports plugins and p2p syncing, and is very much a WIP. My proof of concept is almost done though, which demos the critical technologies in a secure way.
There's a number of Anki alternatives such as https://mnemosyne-proj.org/, but Anki has an active collaborative community, as opposed to most of alternatives, as far as I know.