I recommend the Longman Dictionary of Contemporary English
You can save the dictionary to your Android home screen by following these directions
Add it to your iPhone or iPad by following these directions
Use the app Pocket to collect articles you want to read later, and convert them to listening exercises
For synonyms, antonyms, etc., try Thesaurus.com, Wordhippo, or Reverso
The Language Portal of Canada (government site with lots of information about using English)
Podcasts
There is/are + Back from the market Quizlet deck
There is/are explanation
BBC Six-Minute English: Optimists vs Pessimists
Street Interviews: Are you an optimist or a pessimist