1. Duolingo: A great app if you are interested in languages. It offers quite a few languages. I used it briefly to learn Spanish and liked it very much. I intend to finish that course some day!
2. Memrise: Memrise is a website/app for shared flashcards on many different topics, including languages. Users can also make their own flashcard decks/courses. I am currently using it to expand my vocabulary in English and Arabic.
3. Anki: Great flashcards tool. It is less game-like in that there are no rewards in it. It also gives more control to the users, which makes it a lot more technical than duo lingo and memrise. You can make your own flashcard sets as well as download from the publicly shared ones on the website. I used it to master the material in my more information-heavy classes, although I could have used it for other classes as well. I am also using it for vocab expansion in English and Arabic.
4. Quizlet: This is also another great flashcards tool. I use both Anki and Quizlet for my shool courses. The main difference between the two is that Anki is designed to facilitate long-term retention, while Quizlet does not have any such algorithm. The great features of Quizlet are that it is very user-friendly and offers different ways of reviewing the cards, including two flashcard games: match and gravity.