Yahoo Dictionary

http://education.yahoo.com/reference/dictionary/

Yahoo Free Dictionary is based on the American Heritage® Dictionary of the English Language. You can find definitions, spellings, and word suggestions in this free online searchable dictionary that features over 200,000 entries, 70,000 audio word pronunciations, and 900 full-page color illustrations.

Further, you can have access to Encyclopedia, Thesaurus, World Factbook, Quotations, Anatomy and English-Spanish, Spanish-English Dictionary.

The drawback of Yahoo Dictionary is that there are too many advertisements on the page that makes reading difficult and slows down the search process.