Словари

• Oxford Advanced Learner's Dictionary
  includes definitions and example sentences for advanced level speakers of English and above.
•  Cambridge Dictionaries
  includes these dictionaries: Advanced Learner's, Learner's, Phrasal Verbs, Idioms, American English, French - English and Spanish - English.
• Macmillan Dictionary 
• OneLook.com
  searches many dictionaries from one place. Includes a unique 'Reverse Dictionary'; type in a definition and they suggest the word.