Firstly I wanted to write about the best online dictionaries, but then I understood you have already heard about that.
That's why I have decided to write about the best online vocabulary quizzes, exercises and word challenges.
Probably, this site www.vocabulary.com/ makes your mouth sore. :D It is a well-known resource, pretty boring, with lots of exercises for training and boosting english vocabulary, which contains Latin and Greek roots.
Are there any other good resources for enriching our vocabulary?
Of course, there are some. I will show some powerful vocabulary builders to you.
First of all, there are some awesome vocabulary exercises from the online dictionary Collins
If you click at the very top, at the black space, at the second word after the word «dictionary» (Quiz), there will be opened a page with lots of different quizzes: grammar quizzes, commonly confused words quizzes, thematic word lists quizzes, as well as exercises in other languages (German grammar exercises, French, Italian, Spanish, and even training Chinese characters. All the exercises are divided into groups and you can easily find what you need.
The next one, my favourite resource is Merriam-Webster www.merriam-webster.com/word-games
You can enrich your vocabulary a lot, doing exercises at this site. Over here is a lot that waits to be done for a studious learner: spelling exercises, weekly challenges of the most difficult words, true or false exercises, name that thing exercises.
Besides, there is one more very practically-applicable, useful and interesting site.
Longman-Pearson Online Dictionary.
Over there is a great deal of amazing exercises, for training vocabulary (idioms, phrasal verbs, collocations, and more) grammar exercises, international exams exercises (CAE, FCE, CPE, IELTS, PTE, TOEIC) and even for training pronunciation and listening skills. Is not it amazing, huh?