ForgetMeNot (available in Google Play Store and F-Droid) is a pretty awesome FOSS flash card/quiz app that I'm currently using for my Korean vocab. It has a lot of options and -- importantly for me -- multiple different kind of tests. Self-testing (basic flash card use), multiple choice (they call it "testing with variants"), and spell check. The ability to "invert" cards, so it shows you either the "question" or the "answer" and you provide the other. The ability to hide the "question", so if you want to turn on text-to-speech for phrases/vocab and have to provide the answer by ear.
It's a very neat app, and is a great replacement to paying for Quizlet, in my opinion.
EDIT: My favorite function, which I could not figure out how to do in Quizlet if it's possible at all, is that you can test yourself on multiple sets! I create a new "deck" for each lesson I do in my Korean workbook, and I like to quiz myself on everything I've learned up to now. In Quizlet, I had to go through each lesson individually. ForgetMeNot let's me press and hold to select as many decks as I want, and it shuffles them all together.
