When I came across this tool I gave a cry "Hurrah! THIS IS IT!"
I mean quizlet. What is it like? - It's easy to use online Flashcard tool with a number of interesting additional features. Initially you have the option to familiarise, learn or test the Keywords, you then have a number of games that can be played which insist on the correct answer being given before proceeding. They are curently testing a new feature using voice recogintion which could be useful for language teachers especially. But it's not free of charge. You can also include images on the cards, which is great for younger students.
The site also allows you to create groups and invite members to join the group so you can keep track of what question sets users have taken and how well they did.
I've been testing this tool for 2 months and can defenitely say it's worth using in ELT.