Quizlet (Web, Android, iOS) is a flashcard and quiz-creating app. You can use 'study sets' made by others, or create and share your own.
Pros:
Very simple interface
Don't have to have an account to study existing sets
Cons:
Text responses have to match answer exactly
Also has paid features and wants you to get them
StudyBlue (Web, iOS, Android) is a flashcard and quizzing app that allows you to create and study flashcards, as well as upload materials. Some professors at WIU utilize it in their classes, and upload flashcards and study guides that you can access there.
Pros:
Very robust - you can choose what kind of quiz you want to take, what cards you want to study, etc.
Cons:
You have to have an account to use it
Content that's free and content you must purchase are misleadingly labeled
Problems in Firefox