Here are some sites offering some of the best dictionaries for your studying needs.
http://thefreedictionary.com – Legal, medical, financial, computer, acronyms, encyclopedia, etcetera
http://www.hyperdictionary.com – An online dictionary and thesaurus
http://www.askoxford.com/ – Oxford Dictionary
http://french-linguistics.co.uk/dictionary/ – English/French Dictionary
www.YourDictionary.com – Need to find the correct spelling of a foreign word or phrase? This resource spans 280 languages. It is also an invaluable English resource, including thesaurus functions and etymologies.