Merriam-Webster Online, which is based on Merriam-Webster's Collegiate Dictionary, Eleventh Edition, provides many useful English learning tools and resources besides the complete A-Z entries of the dictionary.
Features:
• Word of the Day, featuring a new word every day with its pronunciation, definition, etymology as well as an example sentence
• Word for the Wise, a daily two-minute radio program on the English language
• Open Dictionary, where users can submit entries not listed in the dictionary
• Free daily crosswords, word games, spelling quizzes, and Word of the Day podcasts
Site URL: http://www.merriam-webster.com/