If you have older students trying to improve their vocabulary for high stakes tests, you may like to have them use this site both at school from time to time as well as at home. It is not just the one word/one definition question site happily. There are also questions with short reading passages where students must use context to select the correct word. If students pick the wrong word, they have the option to check the built in dictionary.
I would encourage students to create free accounts here so that they can track their progress.
ELL Teacher Pros