Quizlet is a site that allows students to create their own flashcards. It then will shuffle them and allow you to see the front and the back. If your students are using the site to study, just know that many of the cards were done by students so there might be a mistake or two. Nonetheless if you broke a unit into parts for different groups, you would have a nice set when you were done wit the unit. Above is one on the Progressive era.