Merriam-Webster Learner?s Dictionary for the ECCE / ECPE exams

http://www.learnersdictionary.com/

[try the word ?hood?]

Macmillan Dictionary, particularly  for phrasal verbs

[try the phrasal verb ?set up?]

http://www.macmillandictionary.com/

Collins American English Dictionary

also  for phrasal verbs

[try the phrasal verb ?wise up?]

http://www.collinsdictionary.com/dictionary/american

Oxford Collocation Dictionary, for fixed expressions or words that commonly co-occur

[try the word ?prerequisite?]

http://www.ozdic.com/

COCA Corpus of Contemporary American English, how words are actually used

[try the word ?access?

http://corpus.byu.edu/coca/

the free dictionary for AmE / BrE pronunciations

[try the word ?iron? & ?entrepreneur?]

http://www.thefreedictionary.com/

Dictionary.com for think maps

[try the word ?wiener?]

http://dictionary.reference.com/

Online Etymology Dictionary for explanations of what English words meant

[try the word ?cretin? and you?ll be surprised when you see which word it?s derived from]

http://www.etymonline.com/index.php

KDictionaries [Random House Websters] for quick translations of a specific English word into more than 40 languages, including Greek

try the word ?ventriloquist?

http://www.kdictionaries-online.com/

Merriam-Webster Collegiate for difficult explanations and encyclopedic articles

try the word ?ant?

http://www.merriam-webster.com/

[Μανόλης Νίνος]