Anki is cross-platform flashcard app that uses spaced repetition to improve retention. (Link)
Pricing
Anki is free for web, desktop, and Android apps, but the iOS app requires a one-time purchase.
Alternatives
  • None (flashcard apps). Although there are other flashcard apps, none of them come close to Anki's ability to help you review flash cards efficiently. Most lack spaced-repetition which is an immediate deal breaker, since it results in you wasting time reviewing cards that you already know.
  • Zettlekasten. Anki is great for memorizing small chunks of information, but flashcards are not always the right tool for learning. To improve your understanding complicated concepts and build connections between disparate ideas, I recommend using the Zettlekasten notetaking technique. I store my Zettlekasten using Obsidian.

Anki was a game changer for me when I discovered it. I don’t have a naturally good memory, so in technical classes with a large amount of notation and terms I often felt like I was struggling with remembering the terminology more than the concepts. Before Anki, I tried physical flashcards and other flashcard apps, but in each case I always got bogged down by numerous flashcards that I already knew, while I did not review the concepts I needed to study often enough to learn them efficiently.

Anki changed this via its spaced repetition model. In spaced repetition, flashcards are displayed more often while you are learning them and the time between reviews become longer each time you correctly answer them.

Although I adopted the use of Anki for learning topics in my study of math and physics, I have since expanded to many other fields—sometimes pushing Anki’s limits.