I've been pretty obsessed with FSRS in general (tldr: https://github.com/open-spaced-repetition/awesome-fsrs/wiki/...) It's a fantastic new-ish scheduler for spaced repetition - basically a machine learning model which adapts to you, and schedules flash (or anything, really, it's an algorithm) cards according to how well you are personally performing - surfacing data like retention, stability, recall, etc. It's a massive jump over previous "learning algorithms" like

For the past 60d I've been using Anki (a flash card program) and it's FSRS setting to learn my French deck (5000 most common French words) and I'm absolutely zooming. I can already follow a fair chunk of conversational French.

I've also been using the same system to learn Chess more deeply (endgames, tactics, openings) through Chessable and a few other websites that offer FSRS. It's levelled up my chess game a lot

Basically - the thing that hooked me was the data. Being able to see how many cards I've reviewed, how many cards are at 90/80% retention, the stability of every piece of that knowledge, the decay rate, etc... It's really cool.