Quizlet and StudyBlue are two online flashcard sites. A year ago I was very high on StudyBlue but it requires one to have a login/password to see and while I still think, that aside, it is a great review device since it allows for pictures and sound recording, I now prefer Quizlet for the following reasons: 1) it allows everyone to see it whether they have a login/password, it now allows you to upload or search for images 3) you can write in a word and it will give you several choices of definitions or, of course, you can write your own. 4) it even allows you to play a few games with your words/definitions 5) it comes in a number of different languages so would help your ESOL students, 6) it will create short quizzes for you and 7) it can be used on an Android or an iTunes device.
Above is a short video showing you some of the features for Quizlet and below is one telling you how to use it. Followers of my blogs know that my students now use it a lot to study for our tests as you can put in virtually any test in the search engine and get a fairly good review set of virtual cards that someone has done. If you want to make a few changes, it will let you make the e-cards your own and then you can make the changes and share that link with your students. Of course you can also have your students make the cards as well.