Not an AI tool, and arguably the most valuable thing on this list. It is a free, open-source spaced repetition system that shows you a card just before you would have forgotten it. The scheduling algorithm is the product, and it rests on the strongest evidence base in learning science.
Strongest at: retention over months and years. Use a model to draft the cards, proofread them, then let Anki decide when you see each one again.
Worth knowing: an unfriendly interface and a real setup cost. It gives you nothing for the first week and a great deal in the sixth month, which is the opposite of how most software feels and why most people abandon it early. Proofread every generated card: a subtly wrong definition gets drilled in as efficiently as a correct one.
Best for: medicine, law, languages, anatomy, and anything with a large volume of material that must be retained for years rather than weeks.