This is a cool website.

Improve your English vocabulary — and do something good for people who have need.

Free Rice.

It works a little like the old word association tests in school. They give you one word, then you choose the closest equivalent from four others. If you get it right three times in a row, they bump you up a level; if you get a word wrong you go down a level until you are hovering pretty close to the sort of language you use and understand easily.

Note to non-native speakers: as you get higher up in the levels, you will run into more words that are not in common use except by people who study specific subjects (history, science, medicine are the ones I have noted so far).