"A powerful tool to implement the two learning techniques: spaced repetition and active recall in the form of algorithm-based flashcards. Might seem intimidating at first glance, but after some exploration you won’t hesitate to integrate it into your study routine. It's highly customizable to fit your needs since it's open source so many add-ons are available, besides offline availability and syncing between devices (with you all the time). Anki is for free on android and desktop - paid on Appstore only."