Our English tutor has recommended an on-line dictionary and thesaurus – http://www.macmillandictionary.com/
I like it! It is really comprehensive and full of clear explanations of words and meaning shades.
Macmillan has an awesome intelligent thesaurus, which shows only words related to the needed meaning, and does not overwhelm the search result. Also Macmillan regularly publishes new English words based on the constant research of live and trustworthy spoken and written language examples.
Nice service! Worth adding to Favourite links, I think.