THE BASICS :
Chegg Prep -- includes pre-loaded cards along with help to create personal cards. Chegg website also has Study 101, help to study better and more effectively.
ExamTime -- also offers quizzes, notes organizer, study planner and Mind Maps - unique brainstorming tool to help you organize a topic for better understanding. Online and app versions.
Flashcard Machine -- basic service free online, additional fee for app versions.
Quizlet -- offers flashcards as well as other study help such as spelling, learner and test modes. Annual subscription upgrade allows you to add images to cards. Work with other students to create cards for a class or to challenge others to compete as you learn.
StudyBlue -- starts free but many features unavailable without subscription.
Study Stack -- a variety of learning games in addition to flashcards, hangman, crossword, matching and more. Android app only.