The Cambridge English dictionary
(very good for finding collocations: for example, if you want to know which verb or preposition can be used with the word research, just type the word in and look through the examples.)



The Cambridge Dictionary homepage also has dictionaries for American English, idioms and phrasal verbs. 



An American dictionary with thesaurus

Collegiate® Dictionary
Collegiate® Thesaurus

The American Heritage® Dictionary
of the English Language

Fourth Edition

Over 90,000 entries feature 10,000 new words and senses, 70,000 audio
word pronunciations, 900 full-page color illustrations, language notes and
word-root appendixes.



Dictionary   Thesaurus   


Plumb Design Visual Thesaurus


Merriam Webster Medical Dictionary  Merriam Webster logo