See first Select an English Dictionary. Most dictionaries mentioned here also have an app, but most are not free.
DICTIONARIES
(for learners) Cambridge Dictionaries Basic (easy) and Learner's (advanced) (UK, US) | Oxford Advanced Learner’s Dictionary (UK, US) | English - Thai (UK, US) | Merriam-Webster Learner's Dictionary (US, UK)
(for all) Merriam-Webster App (US) | Merriam-Webster Dictionary and Thesaurus (US) |
VISUAL (PICTURE) DICTIONARIES
VISUAL Dictionary Online (US) | Picture Dictionary
DICTIONARIES
(for learners) Cambridge Dictionaries Basic (easy) and Learner's (advanced) (UK, US) | Oxford Advanced Learner’s Dictionary (UK, US) | English - Thai (UK, US) | Merriam-Webster Learner's Dictionary (US, UK)
(for all) Merriam-Webster App (US) | Merriam-Webster Dictionary and Thesaurus (US) |
VISUAL (PICTURE) DICTIONARIES
VISUAL Dictionary Online (US) | Picture Dictionary
COLLOCATIONS & THESAURUS
(for learners) Oxford Online Collocation Dictionary
(for all) Merriam-Webster App | Merriam-Webster Dictionary and Thesaurus | MS Office Thesaurus (Review Tab)
IDIOMS, PHRASES, PROVERBS, etc. English Oxford Living Dictionaries | VOA English in a Minute (videos) | Phrases, sayings, idioms and expressions
SLANG (for all) American English Slang - Lessons (videos) | Online Slang Dictionaries and Language Links | Urban Dictionary | Slang City
VOCABULARY BBC 6 Minute English
(for learners) Oxford Online Collocation Dictionary
(for all) Merriam-Webster App | Merriam-Webster Dictionary and Thesaurus | MS Office Thesaurus (Review Tab)
IDIOMS, PHRASES, PROVERBS, etc. English Oxford Living Dictionaries | VOA English in a Minute (videos) | Phrases, sayings, idioms and expressions
SLANG (for all) American English Slang - Lessons (videos) | Online Slang Dictionaries and Language Links | Urban Dictionary | Slang City
VOCABULARY BBC 6 Minute English