Macmillan Online Dictionary
Useful and user-friendly dictionary. Gives the pronunciation of the words and you can choose between American and British definitions.
Oxford Online Collocations Dictionary
This is a great site and a wonderful resource to use when you are writing or when you want to edit what you have written.
Besides clear explanations of the meaning and use of words, there is lots of other useful information on this dictionary site.
Cambridge Dictionaries Online
Another highly reputable dictionary.
Thesaurus.com
If you want synonyms, you'll find them here.
