Anki
Anki is a very useful system that I wish I used while studying in school and university. It is a very powerful flashcard app that uses advanced spaced repetition techniques to help learn and retain important information.
Anki supports everything that I really need natively: it’s free (except for iOS app), cross-device synchronization, supports some basic formatting for the cards and LaTeX typesetting for mathematical formulas. The cards can be easily organized using a hierarchy of decks and tags. They can be imported or exported through sharing decks, too.
The app is very helpful to me now that I’m refreshing a lot of my mathematical knowledge and study Computer Science books and papers. I’m having a lot of fun studying and building small projects to apply new knowledge, but it’s also very important to retain the majority of it. Everyone’s learning style is certainly different, but I definitely think that the most succesful way for me to study is to memorize the fundamental definitions and ideas from some topic, from which I can usually derive most of the results. It’s also ideal for students, because the app is very easy to use and powerful as is.
While probably not as useful for Mathematics and Computer Science, there are plenty of features that enhance cards generation, such as cloze deletion or image occlusion.
What’s surprising to me is that many people I know (even those in Software Engineering field) have never heard of Anki, despite knowing (and possibly even using) apps like Duolingo. An obvious drawback of Anki is that it doesn’t have a very high quality curated list of decks for at least few topics and it has “DIY” flavor but that’s probably also the reason the app is so useful, too.