Here are a few of the many online dictionaries suitable for instructors and higher level learners. The Oxford and Cambridge websites provide blogs for instructors. The Merriam-Webster site contains quizzes and learner activities.
Advanced learners may enjoy using Oxford Advanced American Dictionary or Merriam-Webster Visual Dictionary Online. In other words, it is both a dictionary and a thesaurus, employing a visually appealing neural net (word web) to demonstrate meanings, word associations and concepts.
Free access:
Longman: http://www.ldoceonline.com/
Merriam-Webster: http://www.learnersdictionary.com/
Oxford: http://www.oxfordlearnersdictionaries.com/
Oxford Advanced: http://www.oxfordlearnersdictionaries.com/definition/american_english/
Cambridge: http://dictionary.cambridge.org/us/dictionary/learner-english/
Merriam-Webster Visual Dictionary: http://visual.merriam-webster.com/
