Quizlet live allows you to take any set of flashcards with 12 or more terms and turn them into a fast-paced, competition among your students. All you need to do is sign up for a free teacher account, select the study deck you want, and click on the new "live" button. It could be a set you've created, or one of the millions of sets already in their database.
Recently, I have been using it to pre-teach vocabulary for a novel we are reading. I assign the students a Quizlet to study for homework and then we play the game at the beginning of class the next day. The students who did their homework are more likely to win, and those that didn't do it are still learning in the process. It's a win-win!
The only downside of the game that I see is that I felt a little tethered to my computer because I had to press "start" every 45-60 seconds to begin the next round, and I like to move around my room. However, I found an amazing solution in the Apple App store! (Also available for Android!) It's called remote mouse, and it allows you to turn any touch screen device into a mouse pad that will control your computer. You have to download the app on both your computer and touch screen device so they will sync, but then it allows you to move around the room and press the start button from anywhere! It's a game changer!