"Study as if you were going to live forever; live as if you were going to die tomorrow."
The user community is a very powerful aspect of Quizlet. The terms and definitions from many popular textbooks are likely to be done and waiting for you. The same applies for many standardized tests. The ability to copy and modify flash card sets will save you time in addition to providing a powerful resource to your students. Quizlet also gives you some management tools such as the ability to create folders and embed flash cards and games on your own website, wiki, or blog.
What makes Quizlet awesome is that everything that's been mentioned in this post can be accomplished easily and for free! Quizlet does offer a couple of paid options including a teacher account that allows you to progress monitor, add your own voice, and add pictures. You can upgrade a free account to a teacher account at any time. Quizlet remains a top recommendation.