You may know Duolingo as the company who makes one of the funnest language learning apps, but a year ago, it also released TinyCards, a flashcard app, on iOS. It took a year, but now the app is available on Android too.
TinyCards is a flashcard app a bit in the vein of Duolingo. It has animations, fun graphics, multiple levels, and progress bars that you need to keep topped up. And there are over 200,000 flashcard decks from varying topics to learn, including Chinese, the periodic table, geography, math, and more. Plus you can import your Duolingo-learned words as a flashcard deck to keep your memory fresh. Or you can create your own flashcards, in case there's a topic you're trying to learn.
TinyCards is free in the Play Store and all the flashcard decks are free too - no in app purchases here. I'm going to give it a try once I recover from all the IFA announcements overload.